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.
switch (evt.action) {
    case "GoToPage":
      PDFViewerApplication.appConfig.toolbar.pageNumber.select();
      break;
    case "Find":
      if (!PDFViewerApplication.supportsIntegratedFind) {
        PDFViewerApplication.findBar.toggle();
      }
      break;
    case "Print":
      PDFViewerApplication.triggerPrinting();
      break;
    case "SaveAs":
      PDFViewerApplication.downloadOrSave();
      break;
  }
}
function webViewerPresentationModeChanged(evt) {