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.
items.pageNumber.value = this.pageLabel;
      this.l10n.get("page_of_pages", { pageNumber, pagesCount }).then(msg => {
        items.numPages.textContent = msg;
      });
    } else {
      items.pageNumber.value = pageNumber;
    }
    items.previous.disabled = pageNumber <= 1;
    items.next.disabled = pageNumber >= pagesCount;
    items.zoomOut.disabled = pageScale <= MIN_SCALE;
    items.zoomIn.disabled = pageScale >= MAX_SCALE;
    this.l10n
      .get("page_scale_percent", { scale: Math.round(pageScale * 10000) / 100 })
      .then(msg => {
        let predefinedValueFound = false;
        for (const option of items.scaleSelect.options) {
          if (option.value !== pageScaleValue) {
            option.selected = false;
            continue;
          }
          option.selected = true;
          predefinedValueFound = true;
        }
        if (!predefinedValueFound) {
          items.customScaleOption.textContent = msg;
          items.customScaleOption.selected = true;
        }
      });
  }
  updateLoadingIndicatorState(loading = false) {