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.
pdfViewer.previousPage();
        handled = true;
        break;
      case 82: 
        PDFViewerApplication.rotatePages(-90);
        break;
    }
  }
  if (!handled && !isViewerInPresentationMode) {
    
    
    
    if (
      (evt.keyCode >= 33 && evt.keyCode <= 40) ||
      (evt.keyCode === 32 && curElementTagName !== "BUTTON")
    ) {
      ensureViewerFocused = true;
    }
  }
  if (ensureViewerFocused && !pdfViewer.containsElement(curElement)) {
    
    
    
    pdfViewer.focus();
  }
  if (handled) {
    evt.preventDefault();
  }
}
function beforeUnload(evt) {