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.
this._unblockDocumentLoadEvent();
    this._hideViewBookmark();
    if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("MOZCENTRAL")) {
      const { container } = this.appConfig.errorWrapper;
      container.hidden = true;
    }
    if (!this.pdfLoadingTask) {
      return;
    }
    if (
      (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) &&
      this.pdfDocument?.annotationStorage.size > 0 &&
      this._annotationStorageModified
    ) {
      try {
        
        await this.save();
      } catch (reason) {
        
      }
    }
    const promises = [];
    promises.push(this.pdfLoadingTask.destroy());
    this.pdfLoadingTask = null;
    if (this.pdfDocument) {
      this.pdfDocument = null;