diff --git a/components/files/file-preview-modal.tsx b/components/files/file-preview-modal.tsx
index 666c8429a..fb8e3ea7a 100644
--- a/components/files/file-preview-modal.tsx
+++ b/components/files/file-preview-modal.tsx
@@ -155,6 +155,11 @@ export function FilePreviewModal({ name, onClose, onDownload, getFileContent }:
   // (text/html, image/svg+xml, ...) in a new tab would execute it in-origin.
   const [canOpenInNewTab, setCanOpenInNewTab] = useState(false);
   const [emlContent, setEmlContent] = useState<ParsedEml | null>(null);
+  // The PDF blob itself, kept alongside its object URL: the pdf.js mobile
+  // viewer needs the bytes, because fetching the blob: URL is blocked by CSP
+  // connect-src (#871). The URL is still what the <iframe> and the
+  // open-in-new-tab fallback use.
+  const [pdfBlob, setPdfBlob] = useState<Blob | null>(null);
 
   // Decide whether to render the PDF in a plain <iframe> (desktop) or with the
   // pdf.js canvas viewer (mobile). navigator.pdfViewerEnabled is the standard
@@ -208,6 +213,7 @@ export function FilePreviewModal({ name, onClose, onDownload, getFileContent }:
     setObjectUrl(null);
     setCanOpenInNewTab(false);
     setEmlContent(null);
+    setPdfBlob(null);
     setLoading(true);
     setError(false);
     setResolvedFileType(getFilePreviewKind(name));
@@ -253,6 +259,7 @@ export function FilePreviewModal({ name, onClose, onDownload, getFileContent }:
           if (!cancelled) {
             setObjectUrl(revokeUrl);
             setCanOpenInNewTab(isMimeTypeSafeForInlinePreview(effectiveType));
+            if (previewType === "pdf") setPdfBlob(typedBlob);
           }
         }
       } catch {
@@ -374,7 +381,7 @@ export function FilePreviewModal({ name, onClose, onDownload, getFileContent }:
           />
         )}
 
-        {!loading && !error && fileType === "pdf" && objectUrl && !pdfInlineSupported && (
+        {!loading && !error && fileType === "pdf" && objectUrl && pdfBlob && !pdfInlineSupported && (
           // Mobile browsers can't show a PDF inline in an <iframe> (Android: a
           // blank frame / silent download; iOS: only the first page), so render
           // it with pdf.js (canvas) instead. The header's open-in-new-tab /
@@ -383,7 +390,7 @@ export function FilePreviewModal({ name, onClose, onDownload, getFileContent }:
             className="w-full max-w-3xl h-full overflow-auto rounded-lg bg-neutral-200 dark:bg-neutral-800 p-2"
             onClick={(e) => e.stopPropagation()}
           >
-            <PdfMobileViewer url={objectUrl} />
+            <PdfMobileViewer url={objectUrl} blob={pdfBlob} />
           </div>
         )}
 
diff --git a/components/files/pdf-mobile-viewer.tsx b/components/files/pdf-mobile-viewer.tsx
index fc83baeeb..aab33550c 100644
--- a/components/files/pdf-mobile-viewer.tsx
+++ b/components/files/pdf-mobile-viewer.tsx
@@ -27,7 +27,11 @@ const MAX_ZOOM = 4; // pinch can go a bit beyond the double-tap steps
 const DOUBLE_TAP_MS = 300;
 const DOUBLE_TAP_SLOP = 30; // px
 
-export function PdfMobileViewer({ url }: { url: string }) {
+// `blob` is the document itself; `url` (a blob: URL for the same bytes) is only
+// used for the open-in-new-tab fallback. pdf.js must be fed the bytes, not the
+// URL: fetching a blob: URL is governed by CSP connect-src, which doesn't (and
+// shouldn't need to) allow blob:, so getDocument({ url }) is blocked (#871).
+export function PdfMobileViewer({ url, blob }: { url: string; blob: Blob }) {
   const rootRef = useRef<HTMLDivElement>(null);
   const pagesRef = useRef<HTMLDivElement>(null);
   const zoom = useRef({ step: 0, scale: 1 });
@@ -51,7 +55,12 @@ export function PdfMobileViewer({ url }: { url: string }) {
           import.meta.url,
         ).toString();
 
-        loadingTask = pdfjs.getDocument({ url });
+        // The Uint8Array is created fresh from the blob, so pdf.js taking
+        // ownership of (and transferring) the buffer is fine.
+        const data = new Uint8Array(await blob.arrayBuffer());
+        if (cancelled) return;
+
+        loadingTask = pdfjs.getDocument({ data });
         const doc = await loadingTask.promise;
         if (cancelled) return;
 
@@ -92,7 +101,7 @@ export function PdfMobileViewer({ url }: { url: string }) {
       cancelled = true;
       void loadingTask?.destroy().catch(() => {});
     };
-  }, [url]);
+  }, [blob]);
 
   // Gesture zoom. 1-finger double-tap cycles the steps (fit -> 2x -> 3x ->
   // fit); 2-finger pinch zooms continuously up to MAX_ZOOM. Both drive a