We could use:
https://github.com/mozilla/pdf.js/blob/c3d08580627471432a6ad0b2fdb948df64126afa/src/display/api.js#L929

and look at bit 4.

The following pdf:
https://orimi.com/pdf-test.pdf
is not editable in neither Acrobat nor Edge nor Foxit and nor Okular.
};
    
    loadingTask.onUnsupportedFeature = this.fallback.bind(this);
    return loadingTask.promise.then(
      pdfDocument => {
        this.load(pdfDocument);
      },
      reason => {
        if (loadingTask !== this.pdfLoadingTask) {
          return undefined; 
        }
        let key = "loading_error";
        if (reason instanceof InvalidPDFException) {
          key = "invalid_file_error";
        } else if (reason instanceof MissingPDFException) {
          key = "missing_file_error";
        } else if (reason instanceof UnexpectedResponseException) {
          key = "unexpected_response_error";
        }
        return this.l10n.get(key).then(msg => {
          this._documentError(msg, { message: reason?.message });
          throw reason;
        });
      }
    );
  },
  /**
   * @private
   */
  _ensureDownloadComplete() {