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 82: 
        PDFViewerApplication.rotatePages(90);
        break;
      case 115: 
        PDFViewerApplication.pdfSidebar.toggle();
        break;
    }
    if (
      turnPage !== 0 &&
      (!turnOnlyIfPageFit || pdfViewer.currentScaleValue === "page-fit")
    ) {
      if (turnPage > 0) {
        pdfViewer.nextPage();
      } else {
        pdfViewer.previousPage();
      }
      handled = true;
    }
  }
  
  if (cmd === 4) {
    switch (evt.keyCode) {
      case 13: 
      case 32: 
        if (
          !isViewerInPresentationMode &&
          pdfViewer.currentScaleValue !== "page-fit"
        ) {
          break;
        }