add acroForm check

javascript/src/utilities/pdfUtilities.js

@@ -331,9 +331,9 @@ async function sha256(array) {
 }
 
 function isSigInRoot(pdf) {
-    if (typeof pdf.acroForm === 'undefined') {
-        return false;
-    }
+  if (!pdf.acroForm) {
+    return false;
+  }
     return pdf.acroForm.get('SigFlags') === 3;
 }