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.toolbar.setPageScale(evt.presetValue, evt.scale);
  PDFViewerApplication.pdfViewer.update();
}
function webViewerRotationChanging(evt) {
  PDFViewerApplication.pdfThumbnailViewer.pagesRotation = evt.pagesRotation;
  PDFViewerApplication.forceRendering();
  
  PDFViewerApplication.pdfViewer.currentPageNumber = evt.pageNumber;
}
function webViewerPageChanging({ pageNumber, pageLabel }) {
  PDFViewerApplication.toolbar.setPageNumber(pageNumber, pageLabel);
  PDFViewerApplication.secondaryToolbar.setPageNumber(pageNumber);
  if (PDFViewerApplication.pdfSidebar.visibleView === SidebarView.THUMBS) {
    PDFViewerApplication.pdfThumbnailViewer.scrollThumbnailIntoView(pageNumber);
  }
}
function webViewerVisibilityChange(evt) {
  if (document.visibilityState === "visible") {
    
    setZoomDisabledTimeout();
  }
}
let zoomDisabledTimeout = null;
function setZoomDisabledTimeout() {