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.
PDFViewerApplication.page = PDFViewerApplication.pagesCount;
          handled = true;
          ensureViewerFocused = true;
        }
        break;
    }
  }
  if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC || CHROME")) {
    
    if (cmd === 1 || cmd === 8) {
      switch (evt.keyCode) {
        case 83: 
          eventBus.dispatch("download", { source: window });
          handled = true;
          break;
        case 79: 
          if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
            eventBus.dispatch("openfile", { source: window });
            handled = true;
          }
          break;
      }
    }
  }
  
  if (cmd === 3 || cmd === 10) {
    switch (evt.keyCode) {
      case 80: 
        PDFViewerApplication.requestPresentationMode();
        handled = true;