diff --git a/scripts/test_viewer_pdfjs_resource_cancellation.py b/scripts/test_viewer_pdfjs_resource_cancellation.py
new file mode 100644
index 00000000..8e993a9c
--- /dev/null
+++ b/scripts/test_viewer_pdfjs_resource_cancellation.py
@@ -0,0 +1,185 @@
+#!/usr/bin/env python3
+"""Exercise active PDF.js resource cancellation in a real JavaScript runtime."""
+
+from __future__ import annotations
+
+import shutil
+import subprocess
+from pathlib import Path
+
+
+REPOSITORY_ROOT = Path(__file__).resolve().parents[1]
+VIEWER_SOURCE = REPOSITORY_ROOT / "src/main/resources/static/assets/viewer/viewer.js"
+
+NODE_HARNESS = r"""
+const assert = require("node:assert/strict");
+const fs = require("node:fs");
+const vm = require("node:vm");
+
+function makeElement(tagName = "div") {
+  return {
+    tagName: tagName.toUpperCase(),
+    hidden: false,
+    disabled: false,
+    textContent: "",
+    className: "",
+    href: "",
+    target: "",
+    rel: "",
+    clientWidth: 800,
+    children: [],
+    style: {},
+    attributes: new Map(),
+    setAttribute(name, value) { this.attributes.set(name, String(value)); },
+    getAttribute(name) { return this.attributes.get(name) ?? null; },
+    addEventListener() {},
+    appendChild(child) { this.children.push(child); return child; },
+    querySelector() { return null; },
+    querySelectorAll() { return []; },
+    remove() {},
+    focus() {},
+    getContext() { return {}; },
+  };
+}
+
+function createViewerContext() {
+  const elements = new Map();
+  for (const id of [
+    "doc-meta", "live-status", "error", "error-title", "error-message",
+    "retry-btn", "open-json-link", "preview",
+  ]) {
+    elements.set(id, makeElement());
+  }
+
+  const context = {
+    AbortController,
+    URL,
+    URLSearchParams,
+    console,
+    document: {
+      getElementById(id) { return elements.get(id) ?? null; },
+      querySelector() { return null; },
+      createElement(tagName) { return makeElement(tagName); },
+    },
+    window: {
+      location: { origin: "https://viewer.example.test", search: "" },
+      setTimeout() {},
+      open() { return null; },
+    },
+  };
+  context.globalThis = context;
+  vm.createContext(context);
+  const viewerSource = fs.readFileSync(process.argv[1], "utf8");
+  vm.runInContext(
+    `${viewerSource}\n;globalThis.__viewerTest = { renderPdfInline };`,
+    context,
+    { filename: process.argv[1] },
+  );
+  return context;
+}
+
+async function assertRenderTaskIsCancelled(context) {
+  let resolveRender;
+  let markRenderStarted;
+  const renderPromise = new Promise(resolve => { resolveRender = resolve; });
+  const renderStarted = new Promise(resolve => { markRenderStarted = resolve; });
+  let cancelCalls = 0;
+  let documentDestroyCalls = 0;
+  const renderTask = {
+    promise: renderPromise,
+    cancel() { cancelCalls += 1; },
+  };
+  const pdfDocument = {
+    numPages: 1,
+    async getPage() {
+      return {
+        getViewport({ scale }) { return { width: 100 * scale, height: 200 * scale }; },
+        render() {
+          markRenderStarted();
+          return renderTask;
+        },
+      };
+    },
+    async destroy() { documentDestroyCalls += 1; },
+  };
+  context.__pdfJs = {
+    GlobalWorkerOptions: {},
+    getDocument() {
+      return { promise: Promise.resolve(pdfDocument), destroy() {} };
+    },
+  };
+  vm.runInContext("pdfJsModulePromise = Promise.resolve(globalThis.__pdfJs);", context);
+
+  const controller = new AbortController();
+  const rendering = context.__viewerTest.renderPdfInline(
+    "/artifacts/document.pdf",
+    controller.signal,
+  );
+  await renderStarted;
+  controller.abort();
+
+  assert.equal(cancelCalls, 1, "supersession must actively cancel the current PDF.js RenderTask");
+  resolveRender();
+  await rendering;
+  assert.equal(documentDestroyCalls, 1, "completed render cleanup must still destroy the PDF document");
+}
+
+async function assertLoadingTaskIsDestroyed(context) {
+  let markLoadingStarted;
+  const loadingStarted = new Promise(resolve => { markLoadingStarted = resolve; });
+  let loadingDestroyCalls = 0;
+  const neverSettles = new Promise(() => {});
+  context.__pdfJs = {
+    GlobalWorkerOptions: {},
+    getDocument() {
+      markLoadingStarted();
+      return {
+        promise: neverSettles,
+        destroy() { loadingDestroyCalls += 1; return Promise.resolve(); },
+      };
+    },
+  };
+  vm.runInContext("pdfJsModulePromise = Promise.resolve(globalThis.__pdfJs);", context);
+
+  const controller = new AbortController();
+  void context.__viewerTest.renderPdfInline(
+    "/artifacts/document.pdf",
+    controller.signal,
+  );
+  await loadingStarted;
+  controller.abort();
+  await Promise.resolve();
+
+  assert.equal(
+    loadingDestroyCalls,
+    1,
+    "supersession must destroy an in-flight PDFDocumentLoadingTask when the API is available",
+  );
+}
+
+(async () => {
+  await assertRenderTaskIsCancelled(createViewerContext());
+  await assertLoadingTaskIsDestroyed(createViewerContext());
+})().catch(error => {
+  console.error(error.stack || String(error));
+  process.exitCode = 1;
+});
+"""
+
+
+def test_pdfjs_tasks_are_actively_cancelled_on_supersession() -> None:
+    """Require active RenderTask cancellation and loading-task destruction."""
+
+    node = shutil.which("node")
+    assert node is not None, "Node.js is required for the viewer runtime regression"
+
+    result = subprocess.run(
+        [node, "-e", NODE_HARNESS, str(VIEWER_SOURCE)],
+        cwd=REPOSITORY_ROOT,
+        capture_output=True,
+        check=False,
+        text=True,
+        timeout=15,
+    )
+
+    assert result.returncode == 0, result.stdout + result.stderr
diff --git a/src/main/resources/static/assets/viewer/viewer.js b/src/main/resources/static/assets/viewer/viewer.js
index 33ec0539..e013a5fb 100644
--- a/src/main/resources/static/assets/viewer/viewer.js
+++ b/src/main/resources/static/assets/viewer/viewer.js
@@ -150,8 +150,47 @@ async function renderPdfInline(path, abortSignal) {
     url: resolvedPath,
     withCredentials: true,
   });
-  const pdfDocument = await loadingTask.promise;
+  let pdfDocument = null;
+  let renderTask = null;
+  let cancellationStarted = false;
+  const cancelActivePdfWork = () => {
+    if (cancellationStarted) {
+      return;
+    }
+    cancellationStarted = true;
+
+    if (renderTask && typeof renderTask.cancel === "function") {
+      try {
+        renderTask.cancel();
+      } catch (_error) {
+        // Cancellation is best-effort; abort checks still prevent stale publication.
+      }
+    }
+
+    if (!pdfDocument && typeof loadingTask.destroy === "function") {
+      try {
+        Promise.resolve(loadingTask.destroy()).catch(() => {});
+      } catch (_error) {
+        // Loading-task destruction is best-effort; cancellation remains fail-safe.
+      }
+    }
+  };
+
+  abortSignal?.addEventListener("abort", cancelActivePdfWork, { once: true });
+  if (abortSignal?.aborted) {
+    cancelActivePdfWork();
+  }
+
   try {
+    try {
+      pdfDocument = await loadingTask.promise;
+    } catch (error) {
+      if (abortSignal?.aborted) {
+        return false;
+      }
+      throw error;
+    }
+
     if (abortSignal?.aborted) {
       return false;
     }
@@ -179,7 +218,15 @@ async function renderPdfInline(path, abortSignal) {
     canvas.setAttribute("role", "img");
     canvas.setAttribute("aria-label", `Rendered first page of a ${pdfDocument.numPages}-page PDF`);
 
-    await page.render({ canvasContext: context, viewport }).promise;
+    renderTask = page.render({ canvasContext: context, viewport });
+    try {
+      await renderTask.promise;
+    } catch (error) {
+      if (abortSignal?.aborted) {
+        return false;
+      }
+      throw error;
+    }
     if (abortSignal?.aborted) {
       return false;
     }
@@ -194,7 +241,16 @@ async function renderPdfInline(path, abortSignal) {
     el.preview.appendChild(metadata);
     return true;
   } finally {
-    await pdfDocument.destroy();
+    abortSignal?.removeEventListener("abort", cancelActivePdfWork);
+    if (pdfDocument) {
+      try {
+        await pdfDocument.destroy();
+      } catch (error) {
+        if (!abortSignal?.aborted) {
+          throw error;
+        }
+      }
+    }
   }
 }