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.
case 70: 
        if (!PDFViewerApplication.supportsIntegratedFind && !evt.shiftKey) {
          PDFViewerApplication.findBar.open();
          handled = true;
        }
        break;
      case 71: 
        if (!PDFViewerApplication.supportsIntegratedFind) {
          const { state } = PDFViewerApplication.findController;
          if (state) {
            const eventState = Object.assign(Object.create(null), state, {
              source: window,
              type: "again",
              findPrevious: cmd === 5 || cmd === 12,
            });
            eventBus.dispatch("find", eventState);
          }
          handled = true;
        }
        break;
      case 61: 
      case 107: 
      case 187: 
      case 171: 
        if (!isViewerInPresentationMode) {
          PDFViewerApplication.zoomIn();
        }
        handled = true;
        break;
      case 173: