diff --git a/toolkit/components/pdfjs/content/PdfStreamConverter.sys.mjs b/toolkit/components/pdfjs/content/PdfStreamConverter.sys.mjs
index bf64528d51fd..f049a3824c13 100644
--- a/toolkit/components/pdfjs/content/PdfStreamConverter.sys.mjs
+++ b/toolkit/components/pdfjs/content/PdfStreamConverter.sys.mjs
@@ -525,11 +525,17 @@ class ChromeActions {
     if (typeof data.rawQuery === "string") {
       rawQuery = data.rawQuery;
     }
+    // Same for the `entireWord` property.
+    let entireWord = false;
+    if (typeof data.entireWord === "boolean") {
+      entireWord = data.entireWord;
+    }
 
     let actor = getActor(this.domWindow);
     actor?.sendAsyncMessage("PDFJS:Parent:updateControlState", {
       result,
       findPrevious,
+      entireWord,
       matchesCount,
       rawQuery,
     });
diff --git a/toolkit/components/pdfjs/content/PdfjsParent.sys.mjs b/toolkit/components/pdfjs/content/PdfjsParent.sys.mjs
index 3938f2b9490f..fb37c3c6a6df 100644
--- a/toolkit/components/pdfjs/content/PdfjsParent.sys.mjs
+++ b/toolkit/components/pdfjs/content/PdfjsParent.sys.mjs
@@ -15,6 +15,7 @@
 
 import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 import { PdfJsTelemetry } from "resource://pdf.js/PdfJsTelemetry.sys.mjs";
+import { playNotFoundSound } from "resource://gre/modules/FinderSound.sys.mjs";
 
 const lazy = {};
 
@@ -69,6 +70,7 @@ export class PdfjsParent extends JSWindowActorParent {
     super();
     this._boundToFindbar = null;
     this._findFailedString = null;
+    this._lastNotFoundStringLength = 0;
 
     this._updatedPreference();
   }
@@ -414,6 +416,23 @@ export class PdfjsParent extends JSWindowActorParent {
         lazy.SetClipboardSearchString(data.rawQuery);
       }
 
+      let searchLengthened;
+      switch (data.result) {
+        case Ci.nsITypeAheadFind.FIND_NOTFOUND:
+          searchLengthened =
+            data.rawQuery.length > this._lastNotFoundStringLength;
+          this._lastNotFoundStringLength = data.rawQuery.length;
+
+          if (searchLengthened && !data.entireWord) {
+            playNotFoundSound();
+          }
+          break;
+        case Ci.nsITypeAheadFind.FIND_PENDING:
+          break;
+        default:
+          this._lastNotFoundStringLength = 0;
+      }
+
       const matchesCount = this._requestMatchesCount(data.matchesCount);
       fb.onMatchesCountResult(matchesCount);
     });