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 { toolbar, secondaryToolbar } = this.appConfig;
    toolbar.viewBookmark.hidden = true;
    secondaryToolbar.viewBookmarkButton.hidden = true;
  },
  /**
   * @private
   */
  _cancelIdleCallbacks() {
    if (!this._idleCallbacks.size) {
      return;
    }
    for (const callback of this._idleCallbacks) {
      window.cancelIdleCallback(callback);
    }
    this._idleCallbacks.clear();
  },
  /**
   * Closes opened PDF document.
   * @returns {Promise} - Returns the promise, which is resolved when all
   *                      destruction is completed.
   */
  async close() {