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.
if (pdfDocument !== this.pdfDocument) {
      return;
    }
    const { annotationStorage } = pdfDocument;
    annotationStorage.onSetModified = () => {
      window.addEventListener("beforeunload", beforeUnload);
      if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
        this._annotationStorageModified = true;
      }
    };
    annotationStorage.onResetModified = () => {
      window.removeEventListener("beforeunload", beforeUnload);
      if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
        delete this._annotationStorageModified;
      }
    };
  },
  setInitialView(