diff --git a/apps/x/apps/renderer/src/App.tsx b/apps/x/apps/renderer/src/App.tsx
index 915e3516..39cf44e2 100644
--- a/apps/x/apps/renderer/src/App.tsx
+++ b/apps/x/apps/renderer/src/App.tsx
@@ -13,11 +13,10 @@ import { ChatInputWithMentions, type StagedAttachment } from './components/chat-
 import { ChatMessageAttachments } from '@/components/chat-message-attachments'
 import { GraphView, type GraphEdge, type GraphNode } from '@/components/graph-view';
 import { BasesView, type BaseConfig, DEFAULT_BASE_CONFIG } from '@/components/bases-view';
-import { HtmlFileViewer } from '@/components/html-file-viewer';
 import { ImageFileViewer } from '@/components/image-file-viewer';
 import { VideoFileViewer } from '@/components/video-file-viewer';
-import { PdfFileViewer } from '@/components/pdf-file-viewer';
 import { AudioFileViewer } from '@/components/audio-file-viewer';
+import { PersistentViewerCache } from '@/components/persistent-viewer-cache';
 import { useDebounce } from './hooks/use-debounce';
 import { SidebarContentPanel } from '@/components/sidebar-content';
 import { SuggestedTopicsView } from '@/components/suggested-topics-view';
@@ -4722,6 +4721,15 @@ function App() {
                   />
                 </div>
               ) : selectedPath ? (
+                <>
+                {/* Always-mounted persistent cache for HTML/PDF — hidden when active file is something else, so iframes preserve scroll/page/zoom across switches. */}
+                <div
+                  className="flex-1 min-h-0 overflow-hidden"
+                  style={{ display: /\.(html?|pdf)$/i.test(selectedPath) ? 'block' : 'none' }}
+                >
+                  <PersistentViewerCache activePath={selectedPath} />
+                </div>
+                {!/\.(html?|pdf)$/i.test(selectedPath) && (
                 selectedPath.endsWith('.md') ? (
                   <div className="flex-1 min-h-0 flex flex-row overflow-hidden">
                     <div className="flex-1 min-h-0 flex flex-col overflow-hidden">
@@ -4831,10 +4839,6 @@ function App() {
                       />
                     )}
                   </div>
-                ) : selectedPath?.toLowerCase().endsWith('.html') || selectedPath?.toLowerCase().endsWith('.htm') ? (
-                  <div className="flex-1 min-h-0 overflow-hidden">
-                    <HtmlFileViewer path={selectedPath} />
-                  </div>
                 ) : selectedPath && /\.(png|jpe?g|webp|gif|svg|avif|bmp|ico)$/i.test(selectedPath) ? (
                   <div className="flex-1 min-h-0 overflow-hidden">
                     <ImageFileViewer path={selectedPath} />
@@ -4843,10 +4847,6 @@ function App() {
                   <div className="flex-1 min-h-0 overflow-hidden">
                     <VideoFileViewer path={selectedPath} />
                   </div>
-                ) : selectedPath?.toLowerCase().endsWith('.pdf') ? (
-                  <div className="flex-1 min-h-0 overflow-hidden">
-                    <PdfFileViewer path={selectedPath} />
-                  </div>
                 ) : selectedPath && /\.(mp3|wav|m4a|ogg|flac|aac)$/i.test(selectedPath) ? (
                   <div className="flex-1 min-h-0 overflow-hidden">
                     <AudioFileViewer path={selectedPath} />
@@ -4858,6 +4858,8 @@ function App() {
                     </pre>
                   </div>
                 )
+                )}
+                </>
               ) : selectedTask ? (
                 <div className="flex-1 min-h-0 overflow-hidden">
                   <BackgroundTaskDetail
diff --git a/apps/x/apps/renderer/src/components/persistent-viewer-cache.tsx b/apps/x/apps/renderer/src/components/persistent-viewer-cache.tsx
new file mode 100644
index 00000000..a8dfbbc5
--- /dev/null
+++ b/apps/x/apps/renderer/src/components/persistent-viewer-cache.tsx
@@ -0,0 +1,62 @@
+import { useEffect, useState } from 'react'
+import { HtmlFileViewer } from './html-file-viewer'
+import { PdfFileViewer } from './pdf-file-viewer'
+
+const CACHE_LIMIT = 5
+
+function isCacheable(path: string): boolean {
+  const lower = path.toLowerCase()
+  return lower.endsWith('.html') || lower.endsWith('.htm') || lower.endsWith('.pdf')
+}
+
+function renderViewer(path: string): JSX.Element | null {
+  const lower = path.toLowerCase()
+  if (lower.endsWith('.html') || lower.endsWith('.htm')) {
+    return <HtmlFileViewer path={path} />
+  }
+  if (lower.endsWith('.pdf')) {
+    return <PdfFileViewer path={path} />
+  }
+  return null
+}
+
+interface PersistentViewerCacheProps {
+  activePath: string
+}
+
+/**
+ * Keeps recently-opened HTML and PDF viewers mounted in the DOM,
+ * toggling visibility instead of unmounting. This preserves iframe
+ * state (PDF page/zoom, HTML scroll/JS state) across file switches.
+ */
+export function PersistentViewerCache({ activePath }: PersistentViewerCacheProps) {
+  const [mountedPaths, setMountedPaths] = useState<string[]>(() =>
+    isCacheable(activePath) ? [activePath] : []
+  )
+
+  useEffect(() => {
+    if (!isCacheable(activePath)) return
+    setMountedPaths((prev) => {
+      if (prev.includes(activePath)) {
+        // Move to most-recent position
+        return [...prev.filter((p) => p !== activePath), activePath]
+      }
+      const next = [...prev, activePath]
+      return next.length > CACHE_LIMIT ? next.slice(-CACHE_LIMIT) : next
+    })
+  }, [activePath])
+
+  return (
+    <div className="relative h-full w-full">
+      {mountedPaths.map((p) => (
+        <div
+          key={p}
+          className="absolute inset-0"
+          style={{ display: p === activePath ? 'block' : 'none' }}
+        >
+          {renderViewer(p)}
+        </div>
+      ))}
+    </div>
+  )
+}