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._wasLocalized) {
      
      return;
    }
    const { pageNumber, pagesCount, pageScaleValue, pageScale, items } = this;
    if (resetNumPages) {
      if (this.hasPageLabels) {
        items.pageNumber.type = "text";
      } else {
        items.pageNumber.type = "number";
        this.l10n.get("of_pages", { pagesCount }).then(msg => {
          items.numPages.textContent = msg;
        });
      }
      items.pageNumber.max = pagesCount;
    }
    if (this.hasPageLabels) {