diff --git a/services/web/app/src/Features/Compile/ClsiCacheController.js b/services/web/app/src/Features/Compile/ClsiCacheController.js
new file mode 100644
index 0000000000..a8c99a830b
--- /dev/null
+++ b/services/web/app/src/Features/Compile/ClsiCacheController.js
@@ -0,0 +1,191 @@
+const { NotFoundError } = require('../Errors/Errors')
+const {
+  fetchStreamWithResponse,
+  RequestFailedError,
+  fetchJson,
+} = require('@overleaf/fetch-utils')
+const Path = require('path')
+const { pipeline } = require('stream/promises')
+const logger = require('@overleaf/logger')
+const ClsiCacheManager = require('./ClsiCacheManager')
+const CompileController = require('./CompileController')
+const { expressify } = require('@overleaf/promise-utils')
+const ClsiCacheHandler = require('./ClsiCacheHandler')
+const ProjectGetter = require('../Project/ProjectGetter')
+
+/**
+ * Download a file from a specific build on the clsi-cache.
+ *
+ * @param req
+ * @param res
+ * @return {Promise<*>}
+ */
+async function downloadFromCache(req, res) {
+  const { Project_id: projectId, buildId, filename } = req.params
+  const userId = CompileController._getUserIdForCompile(req)
+  const signal = AbortSignal.timeout(60 * 1000)
+  let location, projectName
+  try {
+    ;[{ location }, { name: projectName }] = await Promise.all([
+      ClsiCacheHandler.getOutputFile(
+        projectId,
+        userId,
+        buildId,
+        filename,
+        signal
+      ),
+      ProjectGetter.promises.getProject(projectId, { name: 1 }),
+    ])
+  } catch (err) {
+    if (err instanceof NotFoundError) {
+      // res.sendStatus() sends a description of the status as body.
+      // Using res.status().end() avoids sending that fake body.
+      return res.status(404).end()
+    } else {
+      throw err
+    }
+  }
+
+  const { stream, response } = await fetchStreamWithResponse(location, {
+    signal,
+  })
+  if (req.destroyed) {
+    // The client has disconnected already, avoid trying to write into the broken connection.
+    return
+  }
+
+  for (const key of ['Content-Length', 'Content-Type']) {
+    if (response.headers.has(key)) res.setHeader(key, response.headers.get(key))
+  }
+  const ext = Path.extname(filename)
+  res.attachment(
+    ext === '.pdf'
+      ? `${CompileController._getSafeProjectName({ name: projectName })}.pdf`
+      : filename
+  )
+  try {
+    res.writeHead(response.status)
+    await pipeline(stream, res)
+  } catch (err) {
+    const reqAborted = Boolean(req.destroyed)
+    const streamingStarted = Boolean(res.headersSent)
+    if (!streamingStarted) {
+      if (err instanceof RequestFailedError) {
+        res.sendStatus(err.response.status)
+      } else {
+        res.sendStatus(500)
+      }
+    }
+    if (
+      streamingStarted &&
+      reqAborted &&
+      err.code === 'ERR_STREAM_PREMATURE_CLOSE'
+    ) {
+      // Ignore noisy spurious error
+      return
+    }
+    logger.warn(
+      {
+        err,
+        projectId,
+        location,
+        filename,
+        reqAborted,
+        streamingStarted,
+      },
+      'CLSI-cache proxy error'
+    )
+  }
+}
+
+/**
+ * Prepare a compile response from the clsi-cache.
+ *
+ * @param req
+ * @param res
+ * @return {Promise<void>}
+ */
+async function getLatestBuildFromCache(req, res) {
+  const { Project_id: projectId } = req.params
+  const userId = CompileController._getUserIdForCompile(req)
+  try {
+    const {
+      internal: { location: metaLocation, zone },
+      external: { isUpToDate, allFiles },
+    } = await ClsiCacheManager.getLatestBuildFromCache(
+      projectId,
+      userId,
+      'output.overleaf.json'
+    )
+
+    if (!isUpToDate) return res.sendStatus(410)
+
+    const meta = await fetchJson(metaLocation, {
+      signal: AbortSignal.timeout(5 * 1000),
+    })
+
+    const [, editorId, buildId] = metaLocation.match(
+      /\/build\/([a-f0-9-]+?)-([a-f0-9]+-[a-f0-9]+)\//
+    )
+
+    let baseURL = `/project/${projectId}`
+    if (userId) {
+      baseURL += `/user/${userId}`
+    }
+
+    const { ranges, contentId, clsiServerId, compileGroup, size } = meta
+
+    const outputFiles = allFiles
+      .filter(
+        path => path !== 'output.overleaf.json' && path !== 'output.tar.gz'
+      )
+      .map(path => {
+        const f = {
+          url: `${baseURL}/build/${editorId}-${buildId}/output/${path}`,
+          downloadURL: `/download/project/${projectId}/build/${editorId}-${buildId}/output/cached/${path}`,
+          build: buildId,
+          path,
+          type: path.split('.').pop(),
+        }
+        if (path === 'output.pdf') {
+          Object.assign(f, {
+            size,
+            editorId,
+          })
+          if (clsiServerId !== 'cache') {
+            // Enable PDF caching and attempt to download from VM first.
+            // (clsi VMs do not have the editorId in the path on disk, omit it).
+            Object.assign(f, {
+              url: `${baseURL}/build/${buildId}/output/output.pdf`,
+              ranges,
+              contentId,
+            })
+          }
+        }
+        return f
+      })
+    let { pdfCachingMinChunkSize, pdfDownloadDomain } =
+      await CompileController._getSplitTestOptions(req, res)
+    pdfDownloadDomain += `/zone/${zone}`
+    res.json({
+      fromCache: true,
+      status: 'success',
+      outputFiles,
+      compileGroup,
+      clsiServerId,
+      pdfDownloadDomain,
+      pdfCachingMinChunkSize,
+    })
+  } catch (err) {
+    if (err instanceof NotFoundError) {
+      res.sendStatus(404)
+    } else {
+      throw err
+    }
+  }
+}
+
+module.exports = {
+  downloadFromCache: expressify(downloadFromCache),
+  getLatestBuildFromCache: expressify(getLatestBuildFromCache),
+}
diff --git a/services/web/app/src/Features/Compile/ClsiCacheHandler.js b/services/web/app/src/Features/Compile/ClsiCacheHandler.js
index de6cde7edc..80296f7f8d 100644
--- a/services/web/app/src/Features/Compile/ClsiCacheHandler.js
+++ b/services/web/app/src/Features/Compile/ClsiCacheHandler.js
@@ -1,3 +1,4 @@
+const _ = require('lodash')
 const {
   fetchNothing,
   fetchRedirectWithResponse,
@@ -23,6 +24,13 @@ function validateFilename(filename) {
   }
 }
 
+/**
+ * Clear the cache on all clsi-cache instances.
+ *
+ * @param projectId
+ * @param userId
+ * @return {Promise<void>}
+ */
 async function clearCache(projectId, userId) {
   let path = `/project/${projectId}`
   if (userId) {
@@ -46,6 +54,45 @@ async function clearCache(projectId, userId) {
   )
 }
 
+/**
+ * Get an output file from a specific build.
+ *
+ * @param projectId
+ * @param userId
+ * @param buildId
+ * @param filename
+ * @param signal
+ * @return {Promise<{size: number, zone: string, location: string, lastModified: Date, allFiles: string[]}>}
+ */
+async function getOutputFile(
+  projectId,
+  userId,
+  buildId,
+  filename,
+  signal = AbortSignal.timeout(15_000)
+) {
+  validateFilename(filename)
+  if (!/^[a-f0-9-]+$/.test(buildId)) {
+    throw new InvalidNameError('bad buildId')
+  }
+
+  let path = `/project/${projectId}`
+  if (userId) {
+    path += `/user/${userId}`
+  }
+  path += `/build/${buildId}/search/output/${filename}`
+  return getRedirectWithFallback(projectId, userId, path, signal)
+}
+
+/**
+ * Get an output file from the most recent build.
+ *
+ * @param projectId
+ * @param userId
+ * @param filename
+ * @param signal
+ * @return {Promise<{size: number, zone: string, location: string, lastModified: Date, allFiles: string[]}>}
+ */
 async function getLatestOutputFile(
   projectId,
   userId,
@@ -59,8 +106,34 @@ async function getLatestOutputFile(
     path += `/user/${userId}`
   }
   path += `/latest/output/${filename}`
+  return getRedirectWithFallback(projectId, userId, path, signal)
+}
 
-  for (const { url, zone } of Settings.apis.clsiCache.instances) {
+/**
+ * Request the given path from any of the clsi-cache instances.
+ *
+ * Some of them might be down temporarily. Try the next one until we receive a redirect or 404.
+ *
+ * This function is similar to the Coordinator in the clsi-cache, notable differences:
+ * - all the logic for sorting builds is in clsi-cache (re-used by clsi and web)
+ * - fan-out (1 client performs lookup on many clsi-cache instances) is "central" in clsi-cache, resulting in better connection re-use
+ * - we only cross the k8s cluster boundary via an internal GCLB once ($$$)
+ *
+ * @param projectId
+ * @param userId
+ * @param path
+ * @param signal
+ * @return {Promise<{size: number, zone: string, location: string, lastModified: Date, allFiles: string[]}>}
+ */
+async function getRedirectWithFallback(
+  projectId,
+  userId,
+  path,
+  signal = AbortSignal.timeout(15_000)
+) {
+  // Avoid hitting the same instance first all the time.
+  const instances = _.shuffle(Settings.apis.clsiCache.instances)
+  for (const { url, zone } of instances) {
     const u = new URL(url)
     u.pathname = path
     try {
@@ -92,6 +165,21 @@ async function getLatestOutputFile(
   throw new NotFoundError('nothing cached yet')
 }
 
+/**
+ * Populate the clsi-cache for the given project/user with the provided source
+ *
+ * This is either another project, or a template (id+version).
+ *
+ * @param projectId
+ * @param userId
+ * @param sourceProjectId
+ * @param templateId
+ * @param templateVersionId
+ * @param lastUpdated
+ * @param zone
+ * @param signal
+ * @return {Promise<void>}
+ */
 async function prepareCacheSource(
   projectId,
   userId,
@@ -122,6 +210,7 @@ async function prepareCacheSource(
 
 module.exports = {
   clearCache,
+  getOutputFile,
   getLatestOutputFile,
   prepareCacheSource,
 }
diff --git a/services/web/app/src/Features/Compile/ClsiCacheManager.js b/services/web/app/src/Features/Compile/ClsiCacheManager.js
index 7c1811dd67..1090b9f2d0 100644
--- a/services/web/app/src/Features/Compile/ClsiCacheManager.js
+++ b/services/web/app/src/Features/Compile/ClsiCacheManager.js
@@ -4,15 +4,26 @@ const DocumentUpdaterHandler = require('../DocumentUpdater/DocumentUpdaterHandle
 const ProjectGetter = require('../Project/ProjectGetter')
 const SplitTestHandler = require('../SplitTests/SplitTestHandler')
 
+/**
+ * Get the most recent build and metadata
+ *
+ * Internal: internal metadata; External: fine to send to user as-is.
+ *
+ * @param projectId
+ * @param userId
+ * @param filename
+ * @param signal
+ * @return {Promise<{internal: {zone: string, location: string}, external: {isUpToDate: boolean, lastUpdated: Date, size: number, allFiles: string[]}}>}
+ */
 async function getLatestBuildFromCache(projectId, userId, filename, signal) {
   const [
     { location, lastModified: lastCompiled, zone, size, allFiles },
     lastUpdatedInRedis,
-    { lastUpdated: lastUpdatedInMongo, name: projectName },
+    { lastUpdated: lastUpdatedInMongo },
   ] = await Promise.all([
     ClsiCacheHandler.getLatestOutputFile(projectId, userId, filename, signal),
     DocumentUpdaterHandler.promises.getProjectLastUpdatedAt(projectId),
-    ProjectGetter.promises.getProject(projectId, { lastUpdated: 1, name: 1 }),
+    ProjectGetter.promises.getProject(projectId, { lastUpdated: 1 }),
   ])
 
   const lastUpdated =
@@ -25,7 +36,6 @@ async function getLatestBuildFromCache(projectId, userId, filename, signal) {
     internal: {
       location,
       zone,
-      projectName,
     },
     external: {
       isUpToDate,
@@ -36,6 +46,16 @@ async function getLatestBuildFromCache(projectId, userId, filename, signal) {
   }
 }
 
+/**
+ * Collect metadata and prepare the clsi-cache for the given project.
+ *
+ * @param projectId
+ * @param userId
+ * @param sourceProjectId
+ * @param templateId
+ * @param templateVersionId
+ * @return {Promise<void>}
+ */
 async function prepareClsiCache(
   projectId,
   userId,
diff --git a/services/web/app/src/Features/Compile/CompileController.js b/services/web/app/src/Features/Compile/CompileController.js
index b7726f1d72..bb225c6b79 100644
--- a/services/web/app/src/Features/Compile/CompileController.js
+++ b/services/web/app/src/Features/Compile/CompileController.js
@@ -57,7 +57,7 @@ async function getPdfCachingMinChunkSize(req, res) {
   return parseInt(variant, 10)
 }
 
-const getSplitTestOptions = callbackify(async function (req, res) {
+async function _getSplitTestOptions(req, res) {
   // Use the query flags from the editor request for overriding the split test.
   let query = {}
   try {
@@ -122,7 +122,8 @@ const getSplitTestOptions = callbackify(async function (req, res) {
     enablePdfCaching,
     pdfCachingMinChunkSize,
   }
-})
+}
+const getSplitTestOptionsCb = callbackify(_getSplitTestOptions)
 
 module.exports = CompileController = {
   compile(req, res, next) {
@@ -161,7 +162,7 @@ module.exports = CompileController = {
       options.incrementalCompilesEnabled = true
     }
 
-    getSplitTestOptions(req, res, (err, splitTestOptions) => {
+    getSplitTestOptionsCb(req, res, (err, splitTestOptions) => {
       if (err) return next(err)
       let {
         compileFromClsiCache,
@@ -305,25 +306,20 @@ module.exports = CompileController = {
     )
   },
 
-  _compileAsUser(req, callback) {
-    // callback with userId if per-user, undefined otherwise
-    if (!Settings.disablePerUserCompiles) {
-      const userId = SessionManager.getLoggedInUserId(req.session)
-      callback(null, userId)
-    } else {
-      callback()
-    }
-  }, // do a per-project compile, not per-user
+  _getSplitTestOptions,
 
-  _downloadAsUser(req, callback) {
-    // callback with userId if per-user, undefined otherwise
+  _getUserIdForCompile(req) {
     if (!Settings.disablePerUserCompiles) {
-      const userId = SessionManager.getLoggedInUserId(req.session)
-      callback(null, userId)
-    } else {
-      callback()
+      return SessionManager.getLoggedInUserId(req.session)
     }
-  }, // do a per-project compile, not per-user
+    return null
+  },
+  _compileAsUser(req, callback) {
+    callback(null, CompileController._getUserIdForCompile(req))
+  },
+  _downloadAsUser(req, callback) {
+    callback(null, CompileController._getUserIdForCompile(req))
+  },
 
   downloadPdf(req, res, next) {
     Metrics.inc('pdf-downloads')
@@ -549,7 +545,7 @@ module.exports = CompileController = {
       getImageNameForProject(projectId, (error, imageName) => {
         if (error) return next(error)
 
-        getSplitTestOptions(req, res, (error, splitTestOptions) => {
+        getSplitTestOptionsCb(req, res, (error, splitTestOptions) => {
           if (error) return next(error)
           const { compileFromClsiCache } = splitTestOptions
 
@@ -597,7 +593,7 @@ module.exports = CompileController = {
       getImageNameForProject(projectId, (error, imageName) => {
         if (error) return next(error)
 
-        getSplitTestOptions(req, res, (error, splitTestOptions) => {
+        getSplitTestOptionsCb(req, res, (error, splitTestOptions) => {
           if (error) return next(error)
           const { compileFromClsiCache } = splitTestOptions
 
diff --git a/services/web/app/src/Features/Project/ProjectController.js b/services/web/app/src/Features/Project/ProjectController.js
index 8fd4668468..8d1f59348b 100644
--- a/services/web/app/src/Features/Project/ProjectController.js
+++ b/services/web/app/src/Features/Project/ProjectController.js
@@ -338,6 +338,8 @@ const _ProjectController = {
       'external-socket-heartbeat',
       'full-project-search',
       'null-test-share-modal',
+      'fall-back-to-clsi-cache',
+      'initial-compile-from-clsi-cache',
       'pdf-caching-cached-url-lookup',
       'pdf-caching-mode',
       'pdf-caching-prefetch-large',
diff --git a/services/web/app/src/router.mjs b/services/web/app/src/router.mjs
index 830620506a..1c024fe7ab 100644
--- a/services/web/app/src/router.mjs
+++ b/services/web/app/src/router.mjs
@@ -66,6 +66,7 @@ import _ from 'lodash'
 import { plainTextResponse } from './infrastructure/Response.js'
 import PublicAccessLevels from './Features/Authorization/PublicAccessLevels.js'
 import SocketDiagnostics from './Features/SocketDiagnostics/SocketDiagnostics.mjs'
+import ClsiCacheController from './Features/Compile/ClsiCacheController.js'
 const ClsiCookieManager = ClsiCookieManagerFactory(
   Settings.apis.clsi != null ? Settings.apis.clsi.backendGroupName : undefined
 )
@@ -625,6 +626,18 @@ async function initialize(webRouter, privateApiRouter, publicApiRouter) {
     CompileController.downloadPdf
   )
 
+  webRouter.get(
+    '/project/:Project_id/output/cached/output.overleaf.json',
+    AuthorizationMiddleware.ensureUserCanReadProject,
+    ClsiCacheController.getLatestBuildFromCache
+  )
+
+  webRouter.get(
+    '/download/project/:Project_id/build/:buildId/output/cached/:filename',
+    AuthorizationMiddleware.ensureUserCanReadProject,
+    ClsiCacheController.downloadFromCache
+  )
+
   // PDF Download button for specific build
   webRouter.get(
     /^\/download\/project\/([^/]*)\/build\/([0-9a-f-]+)\/output\/output\.pdf$/,
diff --git a/services/web/cypress/support/shared/commands/compile.ts b/services/web/cypress/support/shared/commands/compile.ts
index 90567d8e74..e6ea7f70dd 100644
--- a/services/web/cypress/support/shared/commands/compile.ts
+++ b/services/web/cypress/support/shared/commands/compile.ts
@@ -43,23 +43,60 @@ const outputFiles = () => {
   ]
 }
 
-export const interceptCompile = (prefix = 'compile', times = 1) => {
-  cy.intercept(
-    { method: 'POST', pathname: '/project/*/compile', times },
-    {
-      body: {
-        status: 'success',
-        clsiServerId: 'foo',
-        compileGroup: 'priority',
-        pdfDownloadDomain: 'https://clsi.test-overleaf.com',
-        outputFiles: outputFiles(),
-      },
-    }
-  ).as(`${prefix}`)
+export const interceptCompile = ({
+  prefix = 'compile',
+  times = 1,
+  cached = false,
+  outputPDFFixture = 'output.pdf',
+} = {}) => {
+  if (cached) {
+    cy.intercept(
+      { pathname: '/project/*/output/cached/output.overleaf.json', times },
+      {
+        body: {
+          fromCache: true,
+          status: 'success',
+          clsiServerId: 'foo',
+          compileGroup: 'priority',
+          pdfDownloadDomain: 'https://clsi.test-overleaf.com',
+          outputFiles: outputFiles(),
+        },
+      }
+    ).as(`${prefix}-cached`)
+    cy.intercept(
+      { method: 'POST', pathname: '/project/*/compile', times },
+      {
+        body: {
+          status: 'unavailable',
+          clsiServerId: 'foo',
+          compileGroup: 'priority',
+          pdfDownloadDomain: 'https://clsi.test-overleaf.com',
+          outputFiles: [],
+        },
+      }
+    ).as(`${prefix}`)
+  } else {
+    cy.intercept(
+      { pathname: '/project/*/output/cached/output.overleaf.json', times },
+      { statusCode: 404 }
+    ).as(`${prefix}-cached`)
+    cy.intercept(
+      { method: 'POST', pathname: '/project/*/compile', times },
+      {
+        body: {
+          status: 'success',
+          clsiServerId: 'foo',
+          compileGroup: 'priority',
+          pdfDownloadDomain: 'https://clsi.test-overleaf.com',
+          outputFiles: outputFiles(),
+        },
+      }
+    ).as(`${prefix}`)
+  }
 
   cy.intercept(
     { pathname: '/build/*/output.pdf', times },
-    { fixture: 'build/output.pdf,null' }
+    { fixture: `build/${outputPDFFixture},null` }
   ).as(`${prefix}-pdf`)
 
   cy.intercept(
@@ -73,12 +110,26 @@ export const interceptCompile = (prefix = 'compile', times = 1) => {
   ).as(`${prefix}-blg`)
 }
 
-export const waitForCompile = ({ prefix = 'compile', pdf = false } = {}) => {
-  cy.wait(`@${prefix}`)
+export const waitForCompile = ({
+  prefix = 'compile',
+  pdf = false,
+  cached = false,
+} = {}) => {
+  if (cached) {
+    cy.wait(`@${prefix}-cached`)
+  } else {
+    cy.wait(`@${prefix}`)
+  }
   cy.wait(`@${prefix}-log`)
+    .its('request.query.clsiserverid')
+    .should('eq', cached ? 'cache' : 'foo') // straight from cache if cached
   cy.wait(`@${prefix}-blg`)
+    .its('request.query.clsiserverid')
+    .should('eq', cached ? 'cache' : 'foo') // straight from cache if cached
   if (pdf) {
     cy.wait(`@${prefix}-pdf`)
+      .its('request.query.clsiserverid')
+      .should('eq', 'foo') // always from VM first
   }
   return cy.wrap(null)
 }
diff --git a/services/web/frontend/js/features/pdf-preview/components/pdf-file-list.tsx b/services/web/frontend/js/features/pdf-preview/components/pdf-file-list.tsx
index 8ed0b86d9b..753aa32805 100644
--- a/services/web/frontend/js/features/pdf-preview/components/pdf-file-list.tsx
+++ b/services/web/frontend/js/features/pdf-preview/components/pdf-file-list.tsx
@@ -24,7 +24,11 @@ function PdfFileList({ fileList }: { fileList: PdfFileDataList }) {
 
       {fileList.top.map(file => (
         <li key={file.path} role="menuitem">
-          <DropdownItem role="link" download={basename(file)} href={file.url}>
+          <DropdownItem
+            role="link"
+            download={basename(file)}
+            href={file.downloadURL || file.url}
+          >
             {file.path}
           </DropdownItem>
         </li>
@@ -36,7 +40,11 @@ function PdfFileList({ fileList }: { fileList: PdfFileDataList }) {
 
       {fileList.other.map(file => (
         <li key={file.path} role="menuitem">
-          <DropdownItem role="link" download={basename(file)} href={file.url}>
+          <DropdownItem
+            role="link"
+            download={basename(file)}
+            href={file.downloadURL || file.url}
+          >
             {file.path}
           </DropdownItem>
         </li>
diff --git a/services/web/frontend/js/features/pdf-preview/util/file-list.ts b/services/web/frontend/js/features/pdf-preview/util/file-list.ts
index 106234f1fe..310fbb55fb 100644
--- a/services/web/frontend/js/features/pdf-preview/util/file-list.ts
+++ b/services/web/frontend/js/features/pdf-preview/util/file-list.ts
@@ -11,14 +11,21 @@ const ignoreFiles = ['output.fls', 'output.fdb_latexmk']
 
 export function buildFileList(
   outputFiles: Map<string, CompileOutputFile>,
-  { clsiServerId, compileGroup, outputFilesArchive }: CompileResponseData
+  {
+    clsiServerId,
+    compileGroup,
+    outputFilesArchive,
+    fromCache = false,
+  }: CompileResponseData
 ): PdfFileDataList {
   const files: PdfFileDataList = { top: [], other: [] }
 
   if (outputFiles) {
     const params = new URLSearchParams()
 
-    if (clsiServerId) {
+    if (fromCache) {
+      params.set('clsiserverid', 'cache')
+    } else if (clsiServerId) {
       params.set('clsiserverid', clsiServerId)
     }
     if (compileGroup) {
diff --git a/services/web/frontend/js/features/pdf-preview/util/output-files.js b/services/web/frontend/js/features/pdf-preview/util/output-files.js
index 0a79786f64..6c93a02368 100644
--- a/services/web/frontend/js/features/pdf-preview/util/output-files.js
+++ b/services/web/frontend/js/features/pdf-preview/util/output-files.js
@@ -19,9 +19,10 @@ export function handleOutputFiles(outputFiles, projectId, data) {
   outputFile.editorId = outputFile.editorId || EDITOR_SESSION_ID
 
   // build the URL for viewing the PDF in the preview UI
-  const params = new URLSearchParams({
-    compileGroup: data.compileGroup,
-  })
+  const params = new URLSearchParams()
+  if (data.compileGroup) {
+    params.set('compileGroup', data.compileGroup)
+  }
 
   if (data.clsiServerId) {
     params.set('clsiserverid', data.clsiServerId)
@@ -37,10 +38,14 @@ export function handleOutputFiles(outputFiles, projectId, data) {
     data.pdfDownloadDomain
   )}?${params}`
 
-  // build the URL for downloading the PDF
-  params.set('popupDownload', 'true') // save PDF download as file
+  if (data.fromCache) {
+    outputFile.pdfDownloadUrl = outputFile.downloadURL
+  } else {
+    // build the URL for downloading the PDF
+    params.set('popupDownload', 'true') // save PDF download as file
 
-  outputFile.pdfDownloadUrl = `/download/project/${projectId}/build/${outputFile.build}/output/output.pdf?${params}`
+    outputFile.pdfDownloadUrl = `/download/project/${projectId}/build/${outputFile.build}/output/output.pdf?${params}`
+  }
 
   return outputFile
 }
diff --git a/services/web/frontend/js/features/pdf-preview/util/pdf-caching-flags.js b/services/web/frontend/js/features/pdf-preview/util/pdf-caching-flags.js
index 8c15dc4550..d0a5a57ae9 100644
--- a/services/web/frontend/js/features/pdf-preview/util/pdf-caching-flags.js
+++ b/services/web/frontend/js/features/pdf-preview/util/pdf-caching-flags.js
@@ -26,3 +26,4 @@ export const prefetchingEnabled = isFlagEnabled('pdf-caching-prefetching')
 export const prefetchLargeEnabled = isFlagEnabled('pdf-caching-prefetch-large')
 export const enablePdfCaching = isFlagEnabled('pdf-caching-mode')
 export const trackPdfDownloadEnabled = isFlagEnabled('track-pdf-download')
+export const useClsiCache = isFlagEnabled('fall-back-to-clsi-cache')
diff --git a/services/web/frontend/js/features/pdf-preview/util/pdf-caching-transport.js b/services/web/frontend/js/features/pdf-preview/util/pdf-caching-transport.js
index cb7433ee52..3bebc980ae 100644
--- a/services/web/frontend/js/features/pdf-preview/util/pdf-caching-transport.js
+++ b/services/web/frontend/js/features/pdf-preview/util/pdf-caching-transport.js
@@ -1,13 +1,14 @@
 import OError from '@overleaf/o-error'
 import { fallbackRequest, fetchRange } from './pdf-caching'
 import { captureException } from '@/infrastructure/error-reporter'
-import { getPdfCachingMetrics } from './metrics'
+import { EDITOR_SESSION_ID, getPdfCachingMetrics } from './metrics'
 import {
   cachedUrlLookupEnabled,
   enablePdfCaching,
   prefetchingEnabled,
   prefetchLargeEnabled,
   trackPdfDownloadEnabled,
+  useClsiCache,
 } from './pdf-caching-flags'
 import { isNetworkError } from '@/utils/is-network-error'
 import { debugConsole } from '@/utils/debugging'
@@ -50,12 +51,20 @@ export function generatePdfCachingTransportFactory() {
     constructor({ url, pdfFile, abortController, handleFetchError }) {
       super(pdfFile.size, new Uint8Array())
       this.url = url
+      pdfFile.ranges = pdfFile.ranges || []
+      pdfFile.editorId = pdfFile.editorId || EDITOR_SESSION_ID
       this.pdfFile = pdfFile
       // Clone the chunks as the objectId field is encoded to a Uint8Array.
-      this.pdfRanges = pdfFile.ranges.map(r => Object.assign({}, r))
+      this.leanPdfRanges = pdfFile.ranges.map(r => Object.assign({}, r))
       this.handleFetchError = handleFetchError
       this.abortController = abortController
       this.startTime = performance.now()
+
+      const params = new URL(url).searchParams
+      // drop no needed params
+      params.delete('enable_pdf_caching')
+      params.delete('verify_chunks')
+      this.queryForChunks = params.toString()
     }
 
     abort() {
@@ -67,7 +76,7 @@ export function generatePdfCachingTransportFactory() {
       const getDebugInfo = () => ({
         // Sentry does not serialize objects in twice nested objects.
         // Move the ranges to the root level to see them in Sentry.
-        pdfRanges: this.pdfRanges,
+        pdfRanges: this.leanPdfRanges,
         pdfFile: Object.assign({}, this.pdfFile, {
           ranges: '[extracted]',
           // Hide prefetched chunks as these include binary blobs.
@@ -83,7 +92,7 @@ export function generatePdfCachingTransportFactory() {
         performance.now() - this.startTime > STALE_OUTPUT_REQUEST_THRESHOLD_MS
       const is404 = err => OError.getFullInfo(err).statusCode === 404
       const isFromOutputPDFRequest = err =>
-        OError.getFullInfo(err).url === this.url
+        OError.getFullInfo(err).url?.includes?.('/output.pdf') === true
 
       // Do not consider "expected 404s" and network errors as pdf caching
       //  failures.
@@ -96,11 +105,68 @@ export function generatePdfCachingTransportFactory() {
         (is404(err) || isNetworkError(err)) &&
         (isStaleOutputRequest() || isFromOutputPDFRequest(err))
 
+      const usesCache = url => {
+        if (!url) return false
+        const u = new URL(url)
+        return (
+          u.pathname.endsWith(
+            `build/${this.pdfFile.editorId}-${this.pdfFile.build}/output/output.pdf`
+          ) && u.searchParams.get('clsiserverid') === 'cache'
+        )
+      }
+      const canTryFromCache = err => {
+        if (!useClsiCache) return false
+        if (!is404(err)) return false
+        return !usesCache(OError.getFullInfo(err).url)
+      }
+      const getOutputPDFURLFromCache = () => {
+        if (usesCache(this.url)) return this.url
+        const u = new URL(this.url)
+        u.searchParams.set('clsiserverid', 'cache')
+        u.pathname = u.pathname.replace(
+          /build\/[a-f0-9-]+\//,
+          `build/${this.pdfFile.editorId}-${this.pdfFile.build}/`
+        )
+        return u.href
+      }
+      const fetchFromCache = async () => {
+        // Try fetching the chunk from clsi-cache
+        const url = getOutputPDFURLFromCache()
+        return fallbackRequest({
+          file: this.pdfFile,
+          url,
+          start,
+          end,
+          abortSignal,
+        })
+          .then(blob => {
+            // Send the next output.pdf request directly to the cache.
+            this.url = url
+            // Only try downloading chunks that were cached previously
+            this.pdfFile.ranges = this.pdfFile.ranges.filter(r =>
+              cachedUrls.has(r.hash)
+            )
+            return blob
+          })
+          .catch(err => {
+            throw OError.tag(
+              new PDFJS.MissingPDFException(),
+              'cache-fallback',
+              {
+                statusCode: OError.getFullInfo(err).statusCode,
+                url: OError.getFullInfo(err).url,
+                err,
+              }
+            )
+          })
+      }
+
       fetchRange({
         url: this.url,
         start,
         end,
         file: this.pdfFile,
+        queryForChunks: this.queryForChunks,
         metrics,
         usageScore,
         cachedUrls,
@@ -109,9 +175,11 @@ export function generatePdfCachingTransportFactory() {
         prefetchLargeEnabled,
         cachedUrlLookupEnabled,
         abortSignal,
+        fallbackToCacheURL: getOutputPDFURLFromCache(),
       })
         .catch(err => {
           if (abortSignal.aborted) return
+          if (canTryFromCache(err)) return fetchFromCache()
           if (isExpectedError(err)) {
             if (is404(err)) {
               // A regular pdf-js request would have seen this 404 as well.
@@ -140,11 +208,13 @@ export function generatePdfCachingTransportFactory() {
             },
           })
           return fallbackRequest({
+            file: this.pdfFile,
             url: this.url,
             start,
             end,
             abortSignal,
           }).catch(err => {
+            if (canTryFromCache(err)) return fetchFromCache()
             if (isExpectedError(err)) {
               throw OError.tag(new PDFJS.MissingPDFException(), 'fallback', {
                 statusCode: OError.getFullInfo(err).statusCode,
diff --git a/services/web/frontend/js/features/pdf-preview/util/pdf-caching.js b/services/web/frontend/js/features/pdf-preview/util/pdf-caching.js
index 0e3a40d730..77e7d35a2b 100644
--- a/services/web/frontend/js/features/pdf-preview/util/pdf-caching.js
+++ b/services/web/frontend/js/features/pdf-preview/util/pdf-caching.js
@@ -21,6 +21,28 @@ const CHUNK_USAGE_THRESHOLD_CACHED = 42
 // 42 * 0.7^11 < 1, aka we keep stale entries around for 11 compiles.
 const CHUNK_USAGE_STALE_DECAY_RATE = 0.7
 
+let cacheFlag = 'default'
+// Work around a Chrome bug: https://issues.chromium.org/issues/40542704
+// Multiple simultaneous requests to same URL with Range header cause failure (block backend returns ERR_CACHE_OPERATION_NOT_SUPPORTED)
+const CACHE_NO_STORE = 'no-store'
+
+/**
+ * @param {string} url
+ * @param {RequestInit} init
+ */
+async function fetchWithCacheFallback(url, init) {
+  try {
+    return await fetch(url, init)
+  } catch (err) {
+    if (init.headers?.has('Range') && init.cache !== CACHE_NO_STORE) {
+      cacheFlag = CACHE_NO_STORE
+      init.cache = CACHE_NO_STORE
+      return await fetch(url, init)
+    }
+    throw err
+  }
+}
+
 /**
  * @param {Object} file
  */
@@ -470,21 +492,30 @@ export function checkChunkResponse(response, estimatedSize, init) {
   }
 }
 
+function getDynamicChunkInit({ file, start, end, signal }) {
+  // Avoid making range request when downloading the PDF file in full.
+  const isFullFile = start === 0 && end === file.size
+  return {
+    cache: cacheFlag,
+    headers: new Headers(
+      isFullFile ? {} : { Range: `bytes=${start}-${end - 1}` }
+    ),
+    signal,
+  }
+}
+
 /**
  *
+ * @param {Object} file
  * @param {string} url
  * @param {number} start
  * @param {number} end
  * @param {AbortSignal} abortSignal
  */
-export async function fallbackRequest({ url, start, end, abortSignal }) {
+export async function fallbackRequest({ file, url, start, end, abortSignal }) {
   try {
-    const init = {
-      cache: 'no-store',
-      headers: { Range: `bytes=${start}-${end - 1}` },
-      signal: abortSignal,
-    }
-    const response = await fetch(url, init)
+    const init = getDynamicChunkInit({ file, start, end, signal: abortSignal })
+    const response = await fetchWithCacheFallback(url, init)
     checkChunkResponse(response, end - start, init)
     return await response.arrayBuffer()
   } catch (e) {
@@ -494,6 +525,7 @@ export async function fallbackRequest({ url, start, end, abortSignal }) {
 
 /**
  *
+ * @param {Object} file
  * @param {string} url
  * @param {number} start
  * @param {number} end
@@ -501,10 +533,24 @@ export async function fallbackRequest({ url, start, end, abortSignal }) {
  * @param {Uint8Array} actual
  * @param {AbortSignal} abortSignal
  */
-async function verifyRange({ url, start, end, metrics, actual, abortSignal }) {
+async function verifyRange({
+  file,
+  url,
+  start,
+  end,
+  metrics,
+  actual,
+  abortSignal,
+}) {
   let expectedRaw
   try {
-    expectedRaw = await fallbackRequest({ url, start, end, abortSignal })
+    expectedRaw = await fallbackRequest({
+      file,
+      url,
+      start,
+      end,
+      abortSignal,
+    })
   } catch (error) {
     throw OError.tag(error, 'cannot verify range', { url, start, end })
   }
@@ -541,9 +587,11 @@ function skipPrefetched(chunks, prefetched, start, end) {
  * @param {Object|Object[]} chunk
  * @param {string} url
  * @param {RequestInit} init
- * @param {Map<string, string>} cachedUrls
+ * @param {Map<string, {url:string,init:RequestInit}>} cachedUrls
  * @param {Object} metrics
  * @param {boolean} cachedUrlLookupEnabled
+ * @param {string} fallbackToCacheURL
+ * @param {Object} file
  */
 async function fetchChunk({
   chunk,
@@ -552,18 +600,21 @@ async function fetchChunk({
   cachedUrls,
   metrics,
   cachedUrlLookupEnabled,
+  fallbackToCacheURL,
+  file,
 }) {
   const estimatedSize = Array.isArray(chunk)
     ? estimateSizeOfMultipartResponse(chunk)
     : chunk.end - chunk.start
 
   const oldUrl = cachedUrls.get(chunk.hash)
-  if (cachedUrlLookupEnabled && chunk.hash && oldUrl && oldUrl !== url) {
+  if (cachedUrlLookupEnabled && chunk.hash && oldUrl && oldUrl.url !== url) {
     // When the clsi server id changes, the content id changes too and as a
     //  result all the browser cache keys (aka urls) get invalidated.
     // We memorize the previous browser cache keys in `cachedUrls`.
     try {
-      const response = await fetch(oldUrl, init)
+      oldUrl.init.signal = init.signal
+      const response = await fetchWithCacheFallback(oldUrl.url, oldUrl.init)
       if (response.status === 200) {
         checkChunkResponse(response, estimatedSize, init)
         metrics.oldUrlHitCount += 1
@@ -577,10 +628,46 @@ async function fetchChunk({
     } catch (e) {
       // Fallback to the latest url.
     }
+    cachedUrls.delete(chunk.hash) // clear cached state
+  }
+  let response
+  try {
+    response = await fetchWithCacheFallback(url, init)
+    checkChunkResponse(response, estimatedSize, init)
+    if (chunk.hash) {
+      delete init.signal // omit the signal from the cache
+      cachedUrls.set(chunk.hash, { url, init })
+    }
+  } catch (err1) {
+    if (chunk.hash) {
+      cachedUrls.delete(chunk.hash)
+    }
+    const isMissing = response?.status === 404 || response?.status === 416
+    const hasOthersCached = cachedUrls.size > 0
+    if (isMissing && hasOthersCached) {
+      // Only try downloading chunks that were cached previously
+      file.ranges = file.ranges.filter(r => cachedUrls.has(r.hash))
+      // Try harder at fetching the chunk, fallback to cache
+      url = fallbackToCacheURL
+      if (chunk.hash) {
+        init = getDynamicChunkInit({
+          file,
+          // skip object id prefix
+          start: chunk.start + chunk.objectId.byteLength,
+          end: chunk.end,
+          signal: init.signal,
+        })
+      }
+      try {
+        response = await fetchWithCacheFallback(url, init)
+        checkChunkResponse(response, estimatedSize, init)
+      } catch (err2) {
+        throw err1
+      }
+    } else {
+      throw err1
+    }
   }
-  const response = await fetch(url, init)
-  checkChunkResponse(response, estimatedSize, init)
-  if (chunk.hash) cachedUrls.set(chunk.hash, url)
   return response
 }
 
@@ -720,6 +807,7 @@ class Timer {
  * @param {number} start
  * @param {number} end
  * @param {Object} file
+ * @param {queryForChunks} start
  * @param {Object} metrics
  * @param {Map} usageScore
  * @param {Map} cachedUrls
@@ -728,12 +816,14 @@ class Timer {
  * @param {boolean} prefetchLargeEnabled
  * @param {boolean} tryOldCachedUrlEnabled
  * @param {AbortSignal} abortSignal
+ * @param {string} fallbackToCacheURL
  */
 export async function fetchRange({
   url,
   start,
   end,
   file,
+  queryForChunks,
   metrics,
   usageScore,
   cachedUrls,
@@ -742,6 +832,7 @@ export async function fetchRange({
   prefetchLargeEnabled,
   cachedUrlLookupEnabled,
   abortSignal,
+  fallbackToCacheURL,
 }) {
   const timer = new Timer()
   timer.startBlockingCompute()
@@ -793,7 +884,7 @@ export async function fetchRange({
       fetchedCount: 1,
       fetchedBytes: size,
     })
-    return fallbackRequest({ url, start, end, abortSignal })
+    return fallbackRequest({ file, url, start, end, abortSignal })
   }
 
   if (prefetchingEnabled) {
@@ -819,8 +910,8 @@ export async function fetchRange({
         chunk: dynamicChunks[0],
         url,
         init: {
-          cache: 'no-store',
-          headers: { Range: `bytes=${byteRanges}` },
+          cache: cacheFlag,
+          headers: new Headers({ Range: `bytes=${byteRanges}` }),
         },
       })
       break
@@ -833,8 +924,10 @@ export async function fetchRange({
           chunk,
           url,
           init: {
-            cache: 'no-store',
-            headers: { Range: `bytes=${chunk.start}-${chunk.end - 1}` },
+            cache: cacheFlag,
+            headers: new Headers({
+              Range: `bytes=${chunk.start}-${chunk.end - 1}`,
+            }),
           },
         })
       })
@@ -844,17 +937,12 @@ export async function fetchRange({
         chunk: dynamicChunks,
         url,
         init: {
-          cache: 'no-store',
-          headers: { Range: `bytes=${byteRanges}` },
+          cache: cacheFlag,
+          headers: new Headers({ Range: `bytes=${byteRanges}` }),
         },
       })
   }
 
-  const params = new URL(url).searchParams
-  // drop no needed params
-  params.delete('enable_pdf_caching')
-  params.delete('verify_chunks')
-  const query = params.toString()
   // The schema of `url` is https://domain/project/:id/user/:id/build/... for
   //  authenticated and https://domain/project/:id/build/... for
   //  unauthenticated users. Cut it before /build/.
@@ -863,7 +951,7 @@ export async function fetchRange({
   const requests = chunks
     .map(chunk => ({
       chunk,
-      url: `${perUserPrefix}/content/${file.contentId}/${chunk.hash}?${query}`,
+      url: `${perUserPrefix}/content/${file.contentId}/${chunk.hash}?${queryForChunks}`,
       init: {},
     }))
     .concat(coalescedDynamicChunks)
@@ -886,6 +974,8 @@ export async function fetchRange({
           cachedUrls,
           metrics,
           cachedUrlLookupEnabled,
+          fallbackToCacheURL,
+          file,
         })
         timer.startBlockingCompute()
         const boundary = getMultipartBoundary(response, chunk)
diff --git a/services/web/frontend/js/shared/context/local-compile-context.tsx b/services/web/frontend/js/shared/context/local-compile-context.tsx
index e673ba237d..4ecb465382 100644
--- a/services/web/frontend/js/shared/context/local-compile-context.tsx
+++ b/services/web/frontend/js/shared/context/local-compile-context.tsx
@@ -38,12 +38,14 @@ import { useFileTreePathContext } from '@/features/file-tree/contexts/file-tree-
 import { useUserSettingsContext } from '@/shared/context/user-settings-context'
 import { useFeatureFlag } from '@/shared/context/split-test-context'
 import { useEditorManagerContext } from '@/features/ide-react/context/editor-manager-context'
+import { getJSON } from '@/infrastructure/fetch-json'
 import { CompileResponseData } from '../../../../types/compile'
 import {
   PdfScrollPosition,
   usePdfScrollPosition,
 } from '@/shared/hooks/use-pdf-scroll-position'
 import { PdfFileDataList } from '@/features/pdf-preview/util/types'
+import { isSplitTestEnabled } from '@/utils/splitTestUtils'
 
 type PdfFile = Record<string, any>
 
@@ -116,7 +118,7 @@ export const LocalCompileProvider: FC = ({ children }) => {
   const { hasPremiumCompile, isProjectOwner } = useEditorContext()
   const { openDocWithId, openDocs, currentDocument } = useEditorManagerContext()
 
-  const { _id: projectId, rootDocId } = useProjectContext()
+  const { _id: projectId, rootDocId, joinedOnce } = useProjectContext()
 
   const { pdfPreviewOpen } = useLayoutContext()
 
@@ -186,6 +188,12 @@ export const LocalCompileProvider: FC = ({ children }) => {
 
   // whether the project has been compiled yet
   const [compiledOnce, setCompiledOnce] = useState(false)
+  // fetch initial compile response from cache
+  const [initialCompileFromCache, setInitialCompileFromCache] = useState(
+    isSplitTestEnabled('initial-compile-from-clsi-cache')
+  )
+  // Compile triggered while fetching the initial compile from cache
+  const upgradeInitialCompileFromCacheRef = useRef(false)
 
   // whether the cache is being cleared
   const [clearingCache, setClearingCache] = useState(false)
@@ -327,13 +335,43 @@ export const LocalCompileProvider: FC = ({ children }) => {
     setEditedSinceCompileStarted(changedAt > 0)
   }, [setEditedSinceCompileStarted, changedAt])
 
+  // try to fetch the last compile result after opening the project, potentially before joining the project.
+  useEffect(() => {
+    if (initialCompileFromCache) {
+      setInitialCompileFromCache(false)
+      setCompiling(true)
+      setCompiledOnce(true)
+      getJSON(`/project/${projectId}/output/cached/output.overleaf.json`)
+        .then((data: any) => {
+          setCompiling(false)
+          setData({
+            ...data,
+            options: compiler.defaultOptions,
+          })
+          if (upgradeInitialCompileFromCacheRef.current) {
+            compilingRef.current = false
+            compiler.compile() // trigger regular compile
+          }
+        })
+        .catch(() => {
+          setCompiling(false)
+          if (upgradeInitialCompileFromCacheRef.current) {
+            compilingRef.current = false
+            compiler.compile() // trigger regular compile
+          } else {
+            setCompiledOnce(false) // trigger auto compile
+          }
+        })
+    }
+  }, [projectId, initialCompileFromCache, compiler])
+
   // always compile the PDF once after opening the project, after the doc has loaded
   useEffect(() => {
-    if (!compiledOnce && currentDocument) {
+    if (!compiledOnce && currentDocument && !initialCompileFromCache) {
       setCompiledOnce(true)
       compiler.compile({ isAutoCompileOnLoad: true })
     }
-  }, [compiledOnce, currentDocument, compiler])
+  }, [compiledOnce, currentDocument, initialCompileFromCache, compiler])
 
   useEffect(() => {
     setHasShortCompileTimeout(
@@ -370,6 +408,7 @@ export const LocalCompileProvider: FC = ({ children }) => {
   // note: this should _only_ run when `data` changes,
   // the other dependencies must all be static
   useEffect(() => {
+    if (!joinedOnce) return // wait for joinProject, it populates the premium flags.
     const abortController = new AbortController()
 
     const recordedActions = recordedActionsRef.current
@@ -519,6 +558,7 @@ export const LocalCompileProvider: FC = ({ children }) => {
       abortController.abort()
     }
   }, [
+    joinedOnce,
     data,
     alphaProgram,
     labsProgram,
@@ -579,9 +619,10 @@ export const LocalCompileProvider: FC = ({ children }) => {
   // start a compile manually
   const startCompile = useCallback(
     options => {
+      upgradeInitialCompileFromCacheRef.current = true
       compiler.compile(options)
     },
-    [compiler]
+    [compiler, upgradeInitialCompileFromCacheRef]
   )
 
   // stop a compile manually
diff --git a/services/web/frontend/js/shared/context/project-context.tsx b/services/web/frontend/js/shared/context/project-context.tsx
index 4e54acaeed..48532e821c 100644
--- a/services/web/frontend/js/shared/context/project-context.tsx
+++ b/services/web/frontend/js/shared/context/project-context.tsx
@@ -29,6 +29,7 @@ const projectFallback = {
 
 export const ProjectProvider: FC = ({ children }) => {
   const [project] = useScopeValue('project')
+  const joinedOnce = !!project
 
   const {
     _id,
@@ -69,6 +70,7 @@ export const ProjectProvider: FC = ({ children }) => {
       trackChangesState,
       mainBibliographyDocId,
       projectSnapshot,
+      joinedOnce,
     }
   }, [
     _id,
@@ -84,6 +86,7 @@ export const ProjectProvider: FC = ({ children }) => {
     trackChangesState,
     mainBibliographyDocId,
     projectSnapshot,
+    joinedOnce,
   ])
 
   return (
diff --git a/services/web/frontend/js/shared/context/types/project-context.tsx b/services/web/frontend/js/shared/context/types/project-context.tsx
index 91419ed06f..aa045d693f 100644
--- a/services/web/frontend/js/shared/context/types/project-context.tsx
+++ b/services/web/frontend/js/shared/context/types/project-context.tsx
@@ -49,6 +49,7 @@ export type ProjectContextValue = {
   }[]
   trackChangesState: boolean | Record<UserId | '__guests__', boolean>
   projectSnapshot: ProjectSnapshot
+  joinedOnce: boolean
 }
 
 export type ProjectContextUpdateValue = Partial<ProjectContextValue>
diff --git a/services/web/test/frontend/components/pdf-preview/pdf-js-viewer.spec.tsx b/services/web/test/frontend/components/pdf-preview/pdf-js-viewer.spec.tsx
index 12ba83cb48..fda113ef4a 100644
--- a/services/web/test/frontend/components/pdf-preview/pdf-js-viewer.spec.tsx
+++ b/services/web/test/frontend/components/pdf-preview/pdf-js-viewer.spec.tsx
@@ -23,7 +23,7 @@ describe('<PdfJSViewer/>', function () {
       <EditorProviders scope={scope}>
         <PdfPreviewProvider>
           <div className="pdf-viewer">
-            <PdfJsViewer url="/build/123/output.pdf" />
+            <PdfJsViewer url="/build/123/output.pdf?clsiserverid=foo" />
           </div>
         </PdfPreviewProvider>
       </EditorProviders>
@@ -68,7 +68,7 @@ describe('<PdfJSViewer/>', function () {
       <EditorProviders scope={scope}>
         <PdfPreviewProvider>
           <div className="pdf-viewer">
-            <PdfJsViewer url="/build/123/output.pdf" />
+            <PdfJsViewer url="/build/123/output.pdf?clsiserverid=foo" />
           </div>
         </PdfPreviewProvider>
       </EditorProviders>
@@ -88,7 +88,7 @@ describe('<PdfJSViewer/>', function () {
       <EditorProviders scope={scope}>
         <PdfPreviewProvider>
           <div className="pdf-viewer">
-            <PdfJsViewer url="/build/123/output.pdf" />
+            <PdfJsViewer url="/build/123/output.pdf?clsiserverid=foo" />
           </div>
         </PdfPreviewProvider>
       </EditorProviders>
diff --git a/services/web/test/frontend/components/pdf-preview/pdf-preview.spec.tsx b/services/web/test/frontend/components/pdf-preview/pdf-preview.spec.tsx
index 245009de74..3f5222f0e0 100644
--- a/services/web/test/frontend/components/pdf-preview/pdf-preview.spec.tsx
+++ b/services/web/test/frontend/components/pdf-preview/pdf-preview.spec.tsx
@@ -34,12 +34,15 @@ describe('<PdfPreview/>', function () {
       'ol-compilesUserContentDomain',
       'https://compiles-user.dev-overleaf.com'
     )
+    window.metaAttributesCache.set('ol-splitTestVariants', {
+      'initial-compile-from-clsi-cache': 'enabled',
+    })
     cy.interceptEvents()
   })
 
   it('renders the PDF preview', function () {
     window.metaAttributesCache.set('ol-preventCompileOnLoad', false)
-    cy.interceptCompile('compile')
+    cy.interceptCompile()
 
     const scope = mockScope()
 
@@ -57,6 +60,58 @@ describe('<PdfPreview/>', function () {
     cy.findByRole('button', { name: 'Recompile' })
   })
 
+  it('uses the cache when available', function () {
+    cy.interceptCompile({ prefix: 'compile', times: 1, cached: true })
+
+    const scope = mockScope()
+
+    cy.mount(
+      <EditorProviders scope={scope}>
+        <div className="pdf-viewer">
+          <PdfPreview />
+        </div>
+      </EditorProviders>
+    )
+
+    // wait for "compile from cache on load" to finish
+    cy.waitForCompile({ pdf: true, cached: true })
+
+    cy.contains('Your Paper')
+  })
+
+  it('uses the cache when available then compiles', function () {
+    cy.interceptCompile({ prefix: 'compile', times: 1, cached: true })
+
+    const scope = mockScope()
+
+    cy.mount(
+      <EditorProviders scope={scope}>
+        <div className="pdf-viewer">
+          <PdfPreview />
+        </div>
+      </EditorProviders>
+    )
+
+    // wait for "compile from cache on load" to finish
+    cy.waitForCompile({ pdf: true, cached: true })
+    cy.contains('Your Paper')
+
+    // Then trigger a new compile
+    cy.interceptCompile({
+      prefix: 'recompile',
+      times: 1,
+      cached: false,
+      outputPDFFixture: 'output-2.pdf',
+    })
+
+    // press the Recompile button => compile
+    cy.findByRole('button', { name: 'Recompile' }).click()
+
+    // wait for compile to finish
+    cy.waitForCompile({ prefix: 'recompile', pdf: true })
+    cy.contains('Modern Authoring Tools for Science')
+  })
+
   it('runs a compile when the Recompile button is pressed', function () {
     cy.interceptCompile()
 
@@ -479,7 +534,7 @@ describe('<PdfPreview/>', function () {
 
       cy.findByRole('button', { name: 'Recompile' }).click()
       cy.waitForCompile({ pdf: true })
-      cy.interceptCompile('recompile')
+      cy.interceptCompile({ prefix: 'recompile' })
 
       cy.intercept('DELETE', '/project/*/output*', {
         statusCode: 204,
diff --git a/services/web/test/frontend/features/source-editor/components/codemirror-editor-table-generator.spec.tsx b/services/web/test/frontend/features/source-editor/components/codemirror-editor-table-generator.spec.tsx
index b391a85222..25b205ed4d 100644
--- a/services/web/test/frontend/features/source-editor/components/codemirror-editor-table-generator.spec.tsx
+++ b/services/web/test/frontend/features/source-editor/components/codemirror-editor-table-generator.spec.tsx
@@ -108,7 +108,7 @@ describe('<CodeMirrorEditor/> Table editor', function () {
     cy.interceptEvents()
 
     cy.interceptMathJax()
-    cy.interceptCompile('compile', Number.MAX_SAFE_INTEGER)
+    cy.interceptCompile({ prefix: 'compile', times: Number.MAX_SAFE_INTEGER })
     cy.intercept('/project/*/doc/*/metadata', { body: {} })
     window.metaAttributesCache.set('ol-preventCompileOnLoad', true)
   })
diff --git a/services/web/types/compile.ts b/services/web/types/compile.ts
index 11767f36ef..541d03149c 100644
--- a/services/web/types/compile.ts
+++ b/services/web/types/compile.ts
@@ -1,6 +1,7 @@
 export type CompileOutputFile = {
   path: string
   url: string
+  downloadURL?: string
   type: string
   build: string
   ranges?: {