diff --git a/web/viewer.js b/web/viewer.js
index 8885162b2..2254731fc 100644
--- a/web/viewer.js
+++ b/web/viewer.js
@@ -203,6 +203,16 @@ function webViewerLoad() {
 
     window.PDFViewerApplication = pdfjsWebApp.PDFViewerApplication;
     window.PDFViewerApplicationOptions = pdfjsWebAppOptions.AppOptions;
+
+    if (typeof PDFJSDev !== 'undefined' && PDFJSDev.test('GENERIC')) {
+      // Give custom implementations of the default viewer a simpler way to
+      // set various `AppOptions`, by dispatching an event once all viewer
+      // files are loaded but *before* the viewer initialization has run.
+      const event = document.createEvent('CustomEvent');
+      event.initCustomEvent('webviewerloaded', true, true, {});
+      document.dispatchEvent(event);
+    }
+
     pdfjsWebApp.PDFViewerApplication.run(config);
   }
 }