diff --git a/apps/yaak-client/components/responseViewers/PdfViewer.tsx b/apps/yaak-client/components/responseViewers/PdfViewer.tsx
index 54a3174b3..87831318e 100644
--- a/apps/yaak-client/components/responseViewers/PdfViewer.tsx
+++ b/apps/yaak-client/components/responseViewers/PdfViewer.tsx
@@ -5,13 +5,11 @@ import type { PDFDocumentProxy } from "pdfjs-dist";
 import { useMemo, useRef, useState } from "react";
 import { Document, Page, pdfjs } from "react-pdf";
 import { useContainerSize } from "@yaakapp-internal/ui";
+import pdfWorkerUrl from "./pdfWorker?worker&url";
 
 // Document can start loading during render, so configure its worker synchronously.
 // Vite bundles the installed PDF.js worker as a local asset, including offline builds.
-pdfjs.GlobalWorkerOptions.workerSrc = new URL(
-  "pdfjs-dist/build/pdf.worker.min.mjs",
-  import.meta.url,
-).toString();
+pdfjs.GlobalWorkerOptions.workerSrc = new URL(pdfWorkerUrl, import.meta.url).toString();
 
 interface Props {
   /** A URL for the body the host already stored. */
diff --git a/apps/yaak-client/components/responseViewers/pdfWorker.ts b/apps/yaak-client/components/responseViewers/pdfWorker.ts
new file mode 100644
index 000000000..25ded5e42
--- /dev/null
+++ b/apps/yaak-client/components/responseViewers/pdfWorker.ts
@@ -0,0 +1,3 @@
+// The worker has its own globals, so it needs the polyfills before PDF.js evaluates
+import "../../polyfills";
+import "pdfjs-dist/build/pdf.worker.min.mjs";
diff --git a/apps/yaak-client/index.html b/apps/yaak-client/index.html
index deaeb4359..b59379cc7 100644
--- a/apps/yaak-client/index.html
+++ b/apps/yaak-client/index.html
@@ -31,6 +31,7 @@
     <div id="cm-portal" class="cm-portal"></div>
     <div id="react-portal"></div>
     <div id="radix-portal" class="cm-portal"></div>
+    <script type="module" src="/polyfills.ts"></script>
     <script type="module" src="/theme.ts"></script>
     <script type="module" src="/font-size.ts"></script>
     <script type="module" src="/font.ts"></script>
diff --git a/apps/yaak-client/polyfills.test.ts b/apps/yaak-client/polyfills.test.ts
new file mode 100644
index 000000000..a15b56383
--- /dev/null
+++ b/apps/yaak-client/polyfills.test.ts
@@ -0,0 +1,167 @@
+import { describe, expect, test, vi } from "vite-plus/test";
+import { getOrInsert, getOrInsertComputed, readableStreamValues, sumPrecise } from "./polyfills";
+
+describe("getOrInsert", () => {
+  test("returns the existing value without replacing it", () => {
+    const map = new Map([["a", 1]]);
+    expect(getOrInsert.call(map, "a", 2)).toBe(1);
+    expect(map.get("a")).toBe(1);
+  });
+
+  test("inserts and returns the value for a missing key", () => {
+    const map = new Map<string, number>();
+    expect(getOrInsert.call(map, "a", 2)).toBe(2);
+    expect(map.get("a")).toBe(2);
+  });
+
+  test("works on a WeakMap", () => {
+    const key = {};
+    const map = new WeakMap<object, number>();
+    expect(getOrInsert.call(map, key, 1)).toBe(1);
+    expect(getOrInsert.call(map, key, 2)).toBe(1);
+  });
+});
+
+describe("getOrInsertComputed", () => {
+  test("returns the existing value without calling the callback", () => {
+    const map = new Map([["a", 1]]);
+    const callback = vi.fn(() => 2);
+    expect(getOrInsertComputed.call(map, "a", callback)).toBe(1);
+    expect(callback).not.toHaveBeenCalled();
+  });
+
+  test("calls the callback with the key and inserts its result", () => {
+    const map = new Map<string, string>();
+    expect(getOrInsertComputed.call(map, "a", (key) => `${String(key)}!`)).toBe("a!");
+    expect(map.get("a")).toBe("a!");
+  });
+
+  test("stores what the callback returns even if the callback set the key", () => {
+    const map = new Map<string, number>();
+    const result = getOrInsertComputed.call(map, "a", () => {
+      map.set("a", 1);
+      return 2;
+    });
+    expect(result).toBe(2);
+    expect(map.get("a")).toBe(2);
+  });
+
+  test("canonicalizes -0 to +0", () => {
+    const map = new Map<number, number>();
+    getOrInsertComputed.call(map, -0, (key) => {
+      expect(Object.is(key, 0)).toBe(true);
+      return 1;
+    });
+    expect(Object.is([...map.keys()][0], 0)).toBe(true);
+  });
+
+  test("throws when the callback is not a function, even for an existing key", () => {
+    const map = new Map([["a", 1]]);
+    // @ts-expect-error testing a non-callable callback
+    expect(() => getOrInsertComputed.call(map, "a", 1)).toThrow(TypeError);
+  });
+
+  test("is installed on Map and WeakMap", () => {
+    const map = new Map<string, number[]>();
+    map.getOrInsertComputed("a", () => []).push(1);
+    map.getOrInsertComputed("a", () => []).push(2);
+    expect(map.get("a")).toEqual([1, 2]);
+
+    const key = {};
+    const weakMap = new WeakMap<object, number>();
+    expect(weakMap.getOrInsertComputed(key, () => 1)).toBe(1);
+    expect(weakMap.getOrInsert(key, 2)).toBe(1);
+  });
+});
+
+describe("sumPrecise", () => {
+  test("sums without intermediate rounding", () => {
+    expect(sumPrecise([1e20, 0.1, -1e20])).toBe(0.1);
+    expect(sumPrecise([0.1, 0.2, 0.3])).toBe(0.6);
+    expect(sumPrecise(Array.from({ length: 10 }, () => 0.1))).toBe(1);
+  });
+
+  test("rounds half to even on the final step", () => {
+    expect(sumPrecise([1, 1e-16, 1e-16])).toBe(1.0000000000000002);
+    expect(sumPrecise([1, 2 ** -53])).toBe(1);
+  });
+
+  test("handles zeroes, infinities and NaN like the spec", () => {
+    expect(Object.is(sumPrecise([]), -0)).toBe(true);
+    expect(Object.is(sumPrecise([-0, -0]), -0)).toBe(true);
+    expect(Object.is(sumPrecise([-0, 0]), 0)).toBe(true);
+    expect(Object.is(sumPrecise([1, -1]), 0)).toBe(true);
+    expect(sumPrecise([1, Infinity])).toBe(Infinity);
+    expect(sumPrecise([1, -Infinity])).toBe(-Infinity);
+    expect(sumPrecise([Infinity, -Infinity])).toBeNaN();
+    expect(sumPrecise([1, NaN])).toBeNaN();
+  });
+
+  test("accepts any iterable and rejects non-numbers", () => {
+    expect(sumPrecise(new Set([1, 2, 3]))).toBe(6);
+    // @ts-expect-error testing a non-number item
+    expect(() => sumPrecise([1, "2"])).toThrow(TypeError);
+  });
+
+  test("is installed on Math", () => {
+    expect(Math.sumPrecise([1, 2, 3])).toBe(6);
+  });
+});
+
+describe("readableStreamValues", () => {
+  const values = readableStreamValues<number>;
+
+  function streamOf(chunks: number[], onCancel?: () => void) {
+    return new ReadableStream<number>({
+      start(controller) {
+        for (const chunk of chunks) controller.enqueue(chunk);
+        controller.close();
+      },
+      cancel: onCancel,
+    });
+  }
+
+  test("yields every chunk and releases the lock", async () => {
+    const stream = streamOf([1, 2, 3]);
+    const seen: number[] = [];
+    for await (const chunk of values.call(stream)) seen.push(chunk);
+    expect(seen).toEqual([1, 2, 3]);
+    expect(stream.locked).toBe(false);
+  });
+
+  test("cancels the stream when the loop exits early", async () => {
+    const onCancel = vi.fn();
+    const stream = new ReadableStream<number>({
+      pull(controller) {
+        controller.enqueue(1);
+      },
+      cancel: onCancel,
+    });
+    for await (const _ of values.call(stream)) break;
+    expect(onCancel).toHaveBeenCalled();
+    expect(stream.locked).toBe(false);
+  });
+
+  test("leaves the stream open with preventCancel", async () => {
+    const onCancel = vi.fn();
+    const stream = streamOf([1, 2], onCancel);
+    for await (const _ of values.call(stream, { preventCancel: true })) break;
+    expect(onCancel).not.toHaveBeenCalled();
+    expect(stream.locked).toBe(false);
+  });
+
+  test("releases the lock and rethrows when cancelling rejects", async () => {
+    const stream = new ReadableStream<number>({
+      pull(controller) {
+        controller.enqueue(1);
+      },
+      cancel() {
+        throw new Error("cancel failed");
+      },
+    });
+    await expect(async () => {
+      for await (const _ of values.call(stream)) break;
+    }).rejects.toThrow("cancel failed");
+    expect(stream.locked).toBe(false);
+  });
+});
diff --git a/apps/yaak-client/polyfills.ts b/apps/yaak-client/polyfills.ts
new file mode 100644
index 000000000..7f6171dfc
--- /dev/null
+++ b/apps/yaak-client/polyfills.ts
@@ -0,0 +1,172 @@
+/**
+ * Built-ins that PDF.js's modern build calls unconditionally but the system WebKit on
+ * older macOS lacks: the upsert methods and `Math.sumPrecise` arrived in Safari 26.2,
+ * and async iteration of `ReadableStream` in Safari 27. Without them the PDF viewer
+ * throws as soon as it renders. Loaded first by `index.html`, and by `pdfWorker.ts`
+ * because the worker has its own globals.
+ *
+ * Each is defined only when missing, so newer engines keep their native versions.
+ */
+
+type KeyedCollection<K, V> = {
+  has(key: K): boolean;
+  get(key: K): V | undefined;
+  set(key: K, value: V): unknown;
+};
+
+/** https://tc39.es/proposal-upsert/#sec-map.prototype.getOrInsert */
+export function getOrInsert<K, V>(this: KeyedCollection<K, V>, key: K, value: V): V {
+  if (this.has(key)) return this.get(key) as V;
+  this.set(key, value);
+  return value;
+}
+
+/** https://tc39.es/proposal-upsert/#sec-map.prototype.getOrInsertComputed */
+export function getOrInsertComputed<K, V>(
+  this: KeyedCollection<K, V>,
+  key: K,
+  callbackfn: (key: K) => V,
+): V {
+  if (typeof callbackfn !== "function") {
+    throw new TypeError("getOrInsertComputed callback is not a function");
+  }
+  if (this.has(key)) return this.get(key) as V;
+  // Map keys are canonicalized, so -0 is stored and passed on as +0
+  const k = (Object.is(key, -0) ? 0 : key) as K;
+  const value = callbackfn(k);
+  // The spec sets after the callback even if the callback inserted the key itself
+  this.set(k, value);
+  return value;
+}
+
+/**
+ * https://tc39.es/proposal-math-sum/
+ *
+ * Shewchuk's exact summation with round-half-even on the final step (the algorithm
+ * behind Python's `math.fsum`). A finite input set whose running sum overflows
+ * partway through is not handled exactly, which PDF.js's byte lengths and widths
+ * never come near.
+ */
+export function sumPrecise(items: Iterable<number>): number {
+  const partials: number[] = [];
+  let allNegativeZero = true;
+  let sawNaN = false;
+  let sawPositiveInfinity = false;
+  let sawNegativeInfinity = false;
+
+  for (const item of items) {
+    if (typeof item !== "number") {
+      throw new TypeError("Math.sumPrecise requires an iterable of numbers");
+    }
+    if (!Object.is(item, -0)) allNegativeZero = false;
+    if (Number.isNaN(item)) {
+      sawNaN = true;
+    } else if (item === Infinity) {
+      sawPositiveInfinity = true;
+    } else if (item === -Infinity) {
+      sawNegativeInfinity = true;
+    } else {
+      let x = item;
+      let i = 0;
+      for (let y of partials) {
+        if (Math.abs(x) < Math.abs(y)) [x, y] = [y, x];
+        const hi = x + y;
+        const lo = y - (hi - x);
+        if (lo !== 0) partials[i++] = lo;
+        x = hi;
+      }
+      partials.length = i;
+      partials.push(x);
+    }
+  }
+
+  if (sawNaN || (sawPositiveInfinity && sawNegativeInfinity)) return NaN;
+  if (sawPositiveInfinity) return Infinity;
+  if (sawNegativeInfinity) return -Infinity;
+  if (allNegativeZero) return -0;
+
+  let n = partials.length;
+  let hi = 0;
+  let lo = 0;
+  if (n > 0) {
+    hi = partials[--n] as number;
+    while (n > 0) {
+      const x = hi;
+      const y = partials[--n] as number;
+      hi = x + y;
+      lo = y - (hi - x);
+      if (lo !== 0) break;
+    }
+    const next = partials[n - 1];
+    if (n > 0 && next != null && ((lo < 0 && next < 0) || (lo > 0 && next > 0))) {
+      const y = lo * 2;
+      const x = hi + y;
+      if (y === x - hi) hi = x;
+    }
+  }
+  return hi === 0 ? 0 : hi;
+}
+
+/** https://streams.spec.whatwg.org/#rs-asynciterator */
+export async function* readableStreamValues<R>(
+  this: ReadableStream<R>,
+  { preventCancel = false }: { preventCancel?: boolean } = {},
+): AsyncGenerator<R, undefined, undefined> {
+  const reader = this.getReader();
+  let finished = false;
+  try {
+    while (true) {
+      const { done, value } = await reader.read();
+      if (done) {
+        finished = true;
+        return;
+      }
+      yield value;
+    }
+  } catch (err) {
+    finished = true;
+    throw err;
+  } finally {
+    // Leaving the loop early (break, return, throw in the body) cancels the stream. The
+    // lock is released even if cancelling rejects, which then propagates as in the spec.
+    try {
+      if (!finished && !preventCancel) await reader.cancel();
+    } finally {
+      reader.releaseLock();
+    }
+  }
+}
+
+function define(target: object, key: PropertyKey, value: unknown) {
+  if (key in target) return;
+  Object.defineProperty(target, key, { value, writable: true, configurable: true });
+}
+
+define(Map.prototype, "getOrInsert", getOrInsert);
+define(Map.prototype, "getOrInsertComputed", getOrInsertComputed);
+define(WeakMap.prototype, "getOrInsert", getOrInsert);
+define(WeakMap.prototype, "getOrInsertComputed", getOrInsertComputed);
+define(Math, "sumPrecise", sumPrecise);
+if (typeof ReadableStream !== "undefined") {
+  define(ReadableStream.prototype, "values", readableStreamValues);
+  define(
+    ReadableStream.prototype,
+    Symbol.asyncIterator,
+    Reflect.get(ReadableStream.prototype, "values"),
+  );
+}
+
+// TypeScript's lib does not describe these yet
+declare global {
+  interface Map<K, V> {
+    getOrInsert(key: K, value: V): V;
+    getOrInsertComputed(key: K, callbackfn: (key: K) => V): V;
+  }
+  interface WeakMap<K extends WeakKey, V> {
+    getOrInsert(key: K, value: V): V;
+    getOrInsertComputed(key: K, callbackfn: (key: K) => V): V;
+  }
+  interface Math {
+    sumPrecise(items: Iterable<number>): number;
+  }
+}
diff --git a/tests/pdfViewer.test.ts b/tests/pdfViewer.test.ts
index 79e53f5ca..95a6e6de6 100644
--- a/tests/pdfViewer.test.ts
+++ b/tests/pdfViewer.test.ts
@@ -93,9 +93,13 @@ test.each(["http://tauri.localhost", "tauri://localhost"])(
     const pdfjsDir = path.dirname(reactPdfRequire.resolve("pdfjs-dist/package.json"));
     const installed = JSON.parse(await readFile(path.join(pdfjsDir, "package.json"), "utf8"));
     expect(version).toBe(installed.version);
-    expect(Buffer.from(worker.source)).toEqual(
-      await readFile(path.join(pdfjsDir, "build/pdf.worker.min.mjs")),
-    );
+    // The worker is bundled with the polyfills older WebKit needs, ahead of PDF.js itself.
+    const source = Buffer.from(worker.source).toString("utf8");
+    const escapedVersion = installed.version.replaceAll(".", "\\.");
+    expect(source).toMatch(new RegExp(`["'\`]${escapedVersion}["'\`]`));
+    const polyfillAt = source.indexOf("getOrInsertComputed callback is not a function");
+    expect(polyfillAt).toBeGreaterThanOrEqual(0);
+    expect(polyfillAt).toBeLessThan(source.indexOf("pdfjs_internal_editor_"));
   },
   30_000,
 );