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.
throw new Error("Not implemented: createScripting");
  }
  static get supportsIntegratedFind() {
    return shadow(this, "supportsIntegratedFind", false);
  }
  static get supportsDocumentFonts() {
    return shadow(this, "supportsDocumentFonts", true);
  }
  static get supportedMouseWheelZoomModifierKeys() {
    return shadow(this, "supportedMouseWheelZoomModifierKeys", {
      ctrlKey: true,
      metaKey: true,
    });
  }
  static get isInAutomation() {
    return shadow(this, "isInAutomation", false);
  }
}
const PDFViewerApplication = {
  initialBookmark: document.location.hash.substring(1),
  _initializedCapability: createPromiseCapability(),
  appConfig: null,
  pdfDocument: null,
  pdfLoadingTask: null,
  printService: null,
  
  pdfViewer: null,