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._pages = [];
    this._currentPageNumber = 1;
    this._currentScale = UNKNOWN_SCALE;
    this._currentScaleValue = null;
    this._pageLabels = null;
    this.#buffer = new PDFPageViewBuffer(DEFAULT_CACHE_SIZE);
    this._location = null;
    this._pagesRotation = 0;
    this._optionalContentConfigPromise = null;
    this._firstPageCapability = createPromiseCapability();
    this._onePageRenderedCapability = createPromiseCapability();
    this._pagesCapability = createPromiseCapability();
    this._scrollMode = ScrollMode.VERTICAL;
    this._previousScrollMode = ScrollMode.UNKNOWN;
    this._spreadMode = SpreadMode.NONE;
    this.#scrollModePageState = {
      previousPageNumber: 1,
      scrollDown: true,