diff --git a/src/__tests__/proxy-document-serve-framing.test.ts b/src/__tests__/proxy-document-serve-framing.test.ts
new file mode 100644
index 000000000..97cdb1d9c
--- /dev/null
+++ b/src/__tests__/proxy-document-serve-framing.test.ts
@@ -0,0 +1,88 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { NextRequest } from "next/server";
+
+/**
+ * The document vault's decrypt-and-serve route is framed same-origin by the
+ * detail preview (`<iframe>` for inline-class PDFs). The proxy's blanket
+ * `X-Frame-Options: DENY` + `frame-ancestors 'none'` would refuse that
+ * embed, and the blanket page CSP overrides whatever the route sets
+ * (middleware headers win) — so the proxy carves out EXACTLY the
+ * `/original` path with a same-origin framing posture and a document CSP
+ * (`default-src 'none'`; deliberately no `sandbox` — Chromium
+ * force-downloads sandboxed PDFs instead of rendering them).
+ *
+ * These tests pin both sides of that carve-out: the serve path gets the
+ * narrow posture, and every neighbouring documents API path keeps the full
+ * DENY posture.
+ */
+
+vi.mock("@/lib/process-type", () => ({
+  shouldRunWeb: () => true,
+}));
+
+import { proxy } from "../proxy";
+
+function setNodeEnv(value: "development" | "production") {
+  vi.stubEnv("NODE_ENV", value);
+}
+
+beforeEach(() => {
+  setNodeEnv("production");
+});
+
+afterEach(() => {
+  vi.unstubAllEnvs();
+});
+
+function makeRequest(pathname: string): NextRequest {
+  return new NextRequest(`http://localhost${pathname}`, {
+    headers: { cookie: "healthlog_session=sess-1" },
+  });
+}
+
+describe("proxy.ts document serve-route framing carve-out", () => {
+  it("allows same-origin framing on the /original serve path", () => {
+    const res = proxy(makeRequest("/api/documents/inbound/doc123/original"));
+    expect(res.headers.get("x-frame-options")).toBe("SAMEORIGIN");
+    const csp = res.headers.get("content-security-policy") ?? "";
+    expect(csp).toContain("frame-ancestors 'self'");
+    expect(csp).toContain("default-src 'none'");
+    // No `sandbox` directive: Chromium downloads sandboxed PDFs instead of
+    // rendering them, which would break the inline preview outright.
+    expect(csp).not.toMatch(/sandbox/);
+    // No script sources of any kind for a served document.
+    expect(csp).not.toMatch(/script-src/);
+  });
+
+  it("keeps the DENY posture on the documents list route", () => {
+    const res = proxy(makeRequest("/api/documents/inbound"));
+    expect(res.headers.get("x-frame-options")).toBe("DENY");
+    expect(res.headers.get("content-security-policy") ?? "").toContain(
+      "frame-ancestors 'none'",
+    );
+  });
+
+  it("keeps the DENY posture on the document detail route", () => {
+    const res = proxy(makeRequest("/api/documents/inbound/doc123"));
+    expect(res.headers.get("x-frame-options")).toBe("DENY");
+  });
+
+  it("does not match nested or suffixed lookalike paths", () => {
+    for (const path of [
+      "/api/documents/inbound/doc123/original/extra",
+      "/api/documents/inbound/doc123/originalx",
+      "/api/documents/inbound//original",
+    ]) {
+      const res = proxy(makeRequest(path));
+      expect(res.headers.get("x-frame-options"), path).toBe("DENY");
+    }
+  });
+
+  it("keeps the DENY posture on app pages, including /documents itself", () => {
+    const res = proxy(makeRequest("/documents"));
+    expect(res.headers.get("x-frame-options")).toBe("DENY");
+    expect(res.headers.get("content-security-policy") ?? "").toContain(
+      "frame-ancestors 'none'",
+    );
+  });
+});
diff --git a/src/components/documents/document-detail-sheet.tsx b/src/components/documents/document-detail-sheet.tsx
index e376ba082..5dabcf5d8 100644
--- a/src/components/documents/document-detail-sheet.tsx
+++ b/src/components/documents/document-detail-sheet.tsx
@@ -87,10 +87,15 @@ function InlinePreview({
         />
       ) : null}
       {isPdf ? (
+        // No `sandbox` attribute by design: Chromium force-downloads PDFs
+        // in sandboxed frames instead of rendering them. The serve
+        // response itself carries `default-src 'none'` +
+        // `frame-ancestors 'self'` + true Content-Type + nosniff, and the
+        // upload policy denies every format whose inline render could
+        // execute script.
         <iframe
           src={src}
           title={title}
-          sandbox=""
           onLoad={() => setLoaded(true)}
           className={cn(
             "border-border h-[55vh] w-full rounded-lg border",
diff --git a/src/proxy.ts b/src/proxy.ts
index 0e51bc8e3..a21ceacce 100644
--- a/src/proxy.ts
+++ b/src/proxy.ts
@@ -297,9 +297,24 @@ export function proxy(request: NextRequest) {
   // Request ID for log correlation
   response.headers.set("x-request-id", requestId);
 
+  // The document vault's decrypt-and-serve route is framed SAME-ORIGIN by
+  // the document detail preview (an <iframe> for inline-class PDFs). The
+  // blanket `X-Frame-Options: DENY` + `frame-ancestors 'none'` below would
+  // refuse that embed, and the blanket page CSP would clobber the route's
+  // own `Content-Security-Policy: sandbox` (middleware headers win over
+  // route headers). Scope: exactly the `/original` path — list/detail API
+  // routes keep the full DENY posture. The replacement CSP keeps the
+  // sandbox (the served bytes are user-uploaded and must never script in
+  // the app origin) and narrows framing to 'self' instead of dropping it.
+  const isDocumentServeRoute =
+    /^\/api\/documents\/inbound\/[^/]+\/original$/.test(pathname);
+
   // Security headers
   response.headers.set("X-Content-Type-Options", "nosniff");
-  response.headers.set("X-Frame-Options", "DENY");
+  response.headers.set(
+    "X-Frame-Options",
+    isDocumentServeRoute ? "SAMEORIGIN" : "DENY",
+  );
   response.headers.set("Referrer-Policy", "strict-origin-when-cross-origin");
   response.headers.set(
     "Permissions-Policy",
@@ -386,9 +401,23 @@ export function proxy(request: NextRequest) {
   // page-load. Avatars now live on the User row and serve from
   // same-origin `/api/user/avatar/{id}`, so `img-src 'self'` covers
   // them.
-  const csp = isDev
-    ? `default-src 'self'; script-src 'self' 'unsafe-eval' 'unsafe-inline'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; connect-src 'self'; font-src 'self';`
-    : `default-src 'self'; script-src 'self' 'nonce-${nonce}'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; connect-src 'self'${aiConnectSrc}${withingsConnectSrc}${whoopConnectSrc}; font-src 'self'; object-src 'none'; base-uri 'self'; form-action 'self'; frame-ancestors 'none'; worker-src 'self'; report-uri ${cspReportEndpoint}; report-to csp-endpoint;`;
+  const csp = isDocumentServeRoute
+    ? // Vault serve route (see the X-Frame-Options note above): the
+      // response is a user-uploaded document, not an app page.
+      // `default-src 'none'` — a served document may load nothing as if
+      // it were a page — and `frame-ancestors 'self'` permits exactly the
+      // same-origin preview iframe. Deliberately NO `sandbox` directive:
+      // Chromium refuses to render PDFs in sandboxed documents (it
+      // force-downloads them), which would kill the inline preview
+      // outright. The load-bearing boundaries for user-uploaded bytes
+      // stay: magic-byte classification at upload, true Content-Type +
+      // `nosniff` at serve (a PDF can never be reinterpreted as HTML),
+      // HTML/SVG denied entirely, and attachment-only posture for every
+      // non-passive format.
+      `default-src 'none'; frame-ancestors 'self';`
+    : isDev
+      ? `default-src 'self'; script-src 'self' 'unsafe-eval' 'unsafe-inline'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; connect-src 'self'; font-src 'self';`
+      : `default-src 'self'; script-src 'self' 'nonce-${nonce}'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; connect-src 'self'${aiConnectSrc}${withingsConnectSrc}${whoopConnectSrc}; font-src 'self'; object-src 'none'; base-uri 'self'; form-action 'self'; frame-ancestors 'none'; worker-src 'self'; report-uri ${cspReportEndpoint}; report-to csp-endpoint;`;
   response.headers.set("Content-Security-Policy", csp);
 
   // Production-only headers. HSTS carries `preload` so the domain stays