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.
pdfViewer.update();
        });
    });
    pagesPromise.then(
      () => {
        this._unblockDocumentLoadEvent();
        this._initializeAutoPrint(pdfDocument, openActionPromise);
      },
      reason => {
        this.l10n.get("loading_error").then(msg => {
          this._documentError(msg, { message: reason?.message });
        });
      }
    );
    onePageRendered.then(data => {
      this.externalServices.reportTelemetry({
        type: "pageInfo",
        timestamp: data.timestamp,
      });
      pdfDocument.getOutline().then(outline => {
        if (pdfDocument !== this.pdfDocument) {
          return; 
        }
        this.pdfOutlineViewer.render({ outline, pdfDocument });
      });
      pdfDocument.getAttachments().then(attachments => {
        if (pdfDocument !== this.pdfDocument) {
          return; 
        }
        this.pdfAttachmentViewer.render({ attachments });