add on (doc) loaded callback

app/src/main/assets/viewer.js

@@ -210,6 +210,7 @@ function loadDocument() {
     }
 
     loadingTask.promise.then(function (newDoc) {
+        channel.onLoaded();
         pdfDoc = newDoc;
         channel.setNumPages(pdfDoc.numPages);
         pdfDoc.getMetadata().then(function (data) {

app/src/main/java/app/grapheneos/pdfviewer/PdfViewer.java

@@ -202,6 +202,10 @@ public class PdfViewer extends AppCompatActivity implements LoaderManager.Loader
             showPasswordPrompt();
         }
 
+        @JavascriptInterface
+        public void onLoaded() {
+        }
+
         @JavascriptInterface
         public String getPassword() {
             return mEncryptedDocumentPassword != null ? mEncryptedDocumentPassword : "";