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.
eventDetails: { mode: AnnotationEditorType.FREETEXT },
      },
      {
        element: options.editorInkButton,
        eventName: "switchannotationeditormode",
        eventDetails: { mode: AnnotationEditorType.INK },
      },
    ];
    if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
      this.buttons.push({ element: options.openFile, eventName: "openfile" });
    }
    this.items = {
      numPages: options.numPages,
      pageNumber: options.pageNumber,
      scaleSelect: options.scaleSelect,
      customScaleOption: options.customScaleOption,
      previous: options.previous,
      next: options.next,
      zoomIn: options.zoomIn,
      zoomOut: options.zoomOut,
      editorNoneButton: options.editorNoneButton,
      editorFreeTextButton: options.editorFreeTextButton,
      editorInkButton: options.editorInkButton,
    };
    this._wasLocalized = false;
    this.reset();
    
    this._bindListeners(options);
  }
  setPageNumber(pageNumber, pageLabel) {