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 visible = this._getVisiblePages();
    const visiblePages = visible.views,
      numVisiblePages = visiblePages.length;
    if (numVisiblePages === 0) {
      return;
    }
    const newCacheSize = Math.max(DEFAULT_CACHE_SIZE, 2 * numVisiblePages + 1);
    this.#buffer.resize(newCacheSize, visible.ids);
    this.renderingQueue.renderHighestPriority(visible);
    const isSimpleLayout =
      this._spreadMode === SpreadMode.NONE &&
      (this._scrollMode === ScrollMode.PAGE ||
        this._scrollMode === ScrollMode.VERTICAL);
    const currentId = this._currentPageNumber;
    let stillFullyVisible = false;
    for (const page of visiblePages) {
      if (page.percent < 100) {
        break;
      }