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.
});
  }
}
function webViewerPageRendered({ pageNumber, error }) {
  
  
  if (pageNumber === PDFViewerApplication.page) {
    PDFViewerApplication.toolbar.updateLoadingIndicatorState(false);
  }
  
  if (PDFViewerApplication.pdfSidebar.visibleView === SidebarView.THUMBS) {
    const pageView = PDFViewerApplication.pdfViewer.getPageView(
       pageNumber - 1
    );
    const thumbnailView = PDFViewerApplication.pdfThumbnailViewer.getThumbnail(
       pageNumber - 1
    );
    if (pageView && thumbnailView) {
      thumbnailView.setImage(pageView);
    }
  }
  if (error) {
    PDFViewerApplication.l10n.get("rendering_error").then(msg => {
      PDFViewerApplication._otherError(msg, error);
    });
  }
  
  PDFViewerApplication._reportDocumentStatsTelemetry();
}
function webViewerPageMode({ mode }) {