https://github.com/mozilla/pdf.js/blob/0df1a5661958df0c4ed2f73b0f3266d825c5c280/src/display/api.js#L710-L719
The return type signature `@returns {Promise<{num: number, gen: number}>}` is mismatched with its definition. (should be `@returns {Promise<number>}`)
if (worker.destroyed) {
    return Promise.reject(new Error("Worker was destroyed"));
  }
  if (pdfDataRangeTransport) {
    source.length = pdfDataRangeTransport.length;
    source.initialData = pdfDataRangeTransport.initialData;
    source.progressiveDone = pdfDataRangeTransport.progressiveDone;
    source.contentDispositionFilename =
      pdfDataRangeTransport.contentDispositionFilename;
  }
  return worker.messageHandler
    .sendWithPromise("GetDocRequest", {
      docId,
      apiVersion:
        typeof PDFJSDev !== "undefined" && !PDFJSDev.test("TESTING")
          ? PDFJSDev.eval("BUNDLE_VERSION")
          : null,
      
      source: {
        data: source.data,