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.
const pdfThumbnailViewer = this.pdfThumbnailViewer;
    pdfThumbnailViewer.setDocument(pdfDocument);
    const storedPromise = (this.store = new ViewHistory(
      pdfDocument.fingerprints[0]
    ))
      .getMultiple({
        page: null,
        zoom: DEFAULT_SCALE_VALUE,
        scrollLeft: "0",
        scrollTop: "0",
        rotation: null,
        sidebarView: SidebarView.UNKNOWN,
        scrollMode: ScrollMode.UNKNOWN,
        spreadMode: SpreadMode.UNKNOWN,
      })
      .catch(() => {
        
        return Object.create(null);
      });
    firstPagePromise.then(pdfPage => {
      this.loadingBar.setWidth(this.appConfig.viewerContainer);
      this._initializeAnnotationStorageCallbacks(pdfDocument);
      Promise.all([
        animationStarted,
        storedPromise,
        pageLayoutPromise,
        pageModePromise,
        openActionPromise,
      ])
        .then(async ([timeStamp, stored, pageLayout, pageMode, openAction]) => {