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.
overlayManager: null,
  
  preferences: null,
  
  toolbar: null,
  
  secondaryToolbar: null,
  
  eventBus: null,
  
  l10n: null,
  isInitialViewSet: false,
  downloadComplete: false,
  isViewerEmbedded: window.parent !== window,
  url: "",
  baseUrl: "",
  _downloadUrl: "",
  externalServices: DefaultExternalServices,
  _boundEvents: Object.create(null),
  documentInfo: null,
  metadata: null,
  _contentDispositionFilename: null,
  _contentLength: null,
  _saveInProgress: false,
  _docStats: null,
  _wheelUnusedTicks: 0,
  _idleCallbacks: new Set(),
  _PDFBug: null,
  
  async initialize(appConfig) {