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 (PDFViewerApplication.supportsIntegratedFind) {
    PDFViewerApplication.externalServices.updateFindMatchesCount(matchesCount);
  } else {
    PDFViewerApplication.findBar.updateResultsCount(matchesCount);
  }
}
function webViewerUpdateFindControlState({
  state,
  previous,
  matchesCount,
  rawQuery,
}) {
  if (PDFViewerApplication.supportsIntegratedFind) {
    PDFViewerApplication.externalServices.updateFindControlState({
      result: state,
      findPrevious: previous,
      matchesCount,
      rawQuery,
    });
  } else {
    PDFViewerApplication.findBar.updateUIState(state, previous, matchesCount);
  }
}
function webViewerScaleChanging(evt) {