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.
const firstView = this.#buf.keys().next().value;
    firstView?.destroy();
    this.#buf.delete(firstView);
  }
}
/**
 * Simple viewer control to display PDF content/pages.
 *
 * @implements {IPDFAnnotationLayerFactory}
 * @implements {IPDFAnnotationEditorLayerFactory}
 * @implements {IPDFStructTreeLayerFactory}
 * @implements {IPDFTextLayerFactory}
 * @implements {IPDFXfaLayerFactory}
 */
class BaseViewer {
  #buffer = null;
  #annotationEditorMode = AnnotationEditorType.NONE;
  #annotationEditorUIManager = null;
  #annotationMode = AnnotationMode.ENABLE_FORMS;
  #previousAnnotationMode = null;
  #enablePermissions = false;
  #previousContainerHeight = 0;
  #scrollModePageState = null;
  #onVisibilityChange = null;
  /**
   * @param {PDFViewerOptions} options
   */
  constructor(options) {