diff --git a/frontend/src/lib/document/viewer/usePdfRenderer.svelte.test.ts b/frontend/src/lib/document/viewer/usePdfRenderer.svelte.test.ts
index 6b48c3f8..c9e07a51 100644
--- a/frontend/src/lib/document/viewer/usePdfRenderer.svelte.test.ts
+++ b/frontend/src/lib/document/viewer/usePdfRenderer.svelte.test.ts
@@ -205,6 +205,32 @@ describe('createPdfRenderer', () => {
 		expect(fakeLoader).toHaveBeenCalledOnce();
 	});
 
+	it('passes a non-null wasmUrl directory (ending in /) to getDocument, not a bare src string', async () => {
+		const getDocument = vi.fn().mockReturnValue({
+			promise: Promise.resolve({ numPages: 1, getPage: vi.fn() })
+		});
+		const lib = {
+			GlobalWorkerOptions: { workerSrc: '' },
+			getDocument,
+			TextLayer: class {
+				render() {
+					return Promise.resolve();
+				}
+				cancel() {}
+			}
+		} as unknown as typeof import('pdfjs-dist');
+		const r = createPdfRenderer(vi.fn().mockResolvedValue([lib, { default: '' }] as const));
+		await r.init();
+		await r.loadDocument('/api/documents/abc/file');
+
+		expect(getDocument).toHaveBeenCalledTimes(1);
+		const arg = getDocument.mock.calls[0][0] as { url?: string; wasmUrl?: string };
+		expect(arg.url).toBe('/api/documents/abc/file');
+		expect(typeof arg.wasmUrl).toBe('string');
+		expect(arg.wasmUrl).not.toBe('');
+		expect(arg.wasmUrl?.endsWith('/')).toBe(true);
+	});
+
 	it('loadDocument sets error and loading=false when getDocument().promise rejects', async () => {
 		const failingLib = {
 			GlobalWorkerOptions: { workerSrc: '' },
diff --git a/frontend/src/lib/document/viewer/usePdfRenderer.svelte.ts b/frontend/src/lib/document/viewer/usePdfRenderer.svelte.ts
index 13315f8e..c21ad2dd 100644
--- a/frontend/src/lib/document/viewer/usePdfRenderer.svelte.ts
+++ b/frontend/src/lib/document/viewer/usePdfRenderer.svelte.ts
@@ -5,6 +5,12 @@ export type LibLoader = () => Promise<readonly [typeof import('pdfjs-dist'), { d
 const defaultLibLoader: LibLoader = () =>
 	Promise.all([import('pdfjs-dist'), import('pdfjs-dist/build/pdf.worker.min.mjs?url')]);
 
+// pdf.js 5.x decodes JBIG2 / CCITTFax / JPEG2000 images via WebAssembly and
+// needs to know where the .wasm modules are served. Must be a directory URL
+// with a trailing slash — pdf.js appends `jbig2.wasm` etc. Served from our own
+// origin by vite-plugin-static-copy (see vite.config.ts). See issue #708.
+const WASM_URL = '/pdfjs-wasm/';
+
 export function createPdfRenderer(libLoader: LibLoader = defaultLibLoader) {
 	// Reactive state — exposed via getters
 	let currentPage = $state(1);
@@ -44,7 +50,7 @@ export function createPdfRenderer(libLoader: LibLoader = defaultLibLoader) {
 		totalPages = 0;
 
 		try {
-			const loadingTask = pdfjsLib.getDocument(src);
+			const loadingTask = pdfjsLib.getDocument({ url: src, wasmUrl: WASM_URL });
 			const doc = await loadingTask.promise;
 			pdfDoc = doc;
 			totalPages = doc.numPages;