diff --git a/test/integration/viewer_spec.mjs b/test/integration/viewer_spec.mjs
index b3b299d74..2a319017e 100644
--- a/test/integration/viewer_spec.mjs
+++ b/test/integration/viewer_spec.mjs
@@ -1359,6 +1359,36 @@ describe("PDF viewer", () => {
     });
   });
 
+  describe("File param with a relative URL and a query string (issue 20137)", () => {
+    let pages;
+
+    beforeEach(async () => {
+      // The `file` parameter is `/test/pdfs/basicapi.pdf?token=%2Ffoo`.
+      pages = await loadAndWait(
+        "basicapi.pdf%3Ftoken%3D%252Ffoo",
+        ".textLayer .endOfContent"
+      );
+    });
+
+    afterEach(async () => {
+      await closePages(pages);
+    });
+
+    it("must not re-encode the file param", async () => {
+      await Promise.all(
+        pages.map(async ([browserName, page]) => {
+          const pdfUrl = await page.evaluate(
+            () => window.PDFViewerApplication.url
+          );
+
+          expect(pdfUrl)
+            .withContext(`In ${browserName}`)
+            .toBe("/test/pdfs/basicapi.pdf?token=%2Ffoo");
+        })
+      );
+    });
+  });
+
   describe("Keyboard scrolling on startup (bug 843653)", () => {
     let pages;
 
diff --git a/web/app.js b/web/app.js
index 12af58ebe..77e2a3f85 100644
--- a/web/app.js
+++ b/web/app.js
@@ -888,11 +888,13 @@ const PDFViewerApplication = {
       const queryString = document.location.search.substring(1);
       const params = parseQueryString(queryString);
       file = params.get("file") ?? AppOptions.get("defaultUrl");
-      try {
-        file = new URL(file).href;
-      } catch {
-        file = encodeURIComponent(file).replaceAll("%2F", "/");
-      }
+      // Note that `parseQueryString` has already percent-decoded the parameter,
+      // hence it's used as-is below: re-encoding it would break URLs with e.g.
+      // a query string or a percent-encoded path (issue 20137).
+      // In a relative URL a "#" is assumed to be part of the filename, rather
+      // than a fragment separator, since the viewer takes its own hash
+      // parameters from the *viewer* URL (issue 19990).
+      file = URL.parse(file)?.href ?? file.replaceAll("#", "%23");
       validateFileURL(file);
     } else if (PDFJSDev.test("MOZCENTRAL")) {
       file = window.location.href;