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._currentPageNumber === val) {
      if (resetCurrentPageView) {
        this.#resetCurrentPageView();
      }
      return true;
    }
    if (!(0 < val && val <= this.pagesCount)) {
      return false;
    }
    const previous = this._currentPageNumber;
    this._currentPageNumber = val;
    this.eventBus.dispatch("pagechanging", {
      source: this,
      pageNumber: val,
      pageLabel: this._pageLabels?.[val - 1] ?? null,
      previous,
    });
    if (resetCurrentPageView) {
      this.#resetCurrentPageView();
    }
    return true;
  }
  /**
   * @type {string|null} Returns the current page label, or `null` if no page
   *   labels exist.
   */
  get currentPageLabel() {