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.
if (this.pdfViewer.isInPresentationMode) {
      return;
    }
    this.pdfViewer.currentScaleValue = DEFAULT_SCALE_VALUE;
  },
  get pagesCount() {
    return this.pdfDocument ? this.pdfDocument.numPages : 0;
  },
  get page() {
    return this.pdfViewer.currentPageNumber;
  },
  set page(val) {
    this.pdfViewer.currentPageNumber = val;
  },
  get supportsPrinting() {
    return PDFPrintServiceFactory.instance.supportsPrinting;
  },
  get supportsFullscreen() {
    return shadow(this, "supportsFullscreen", document.fullscreenEnabled);
  },
  get supportsIntegratedFind() {
    return this.externalServices.supportsIntegratedFind;
  },
  get supportsDocumentFonts() {
    return this.externalServices.supportsDocumentFonts;
  },
  get loadingBar() {