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 (this.pdfDocument) {
      this.eventBus.dispatch("pagesdestroy", { source: this });
      this._cancelRendering();
      this._resetView();
      if (this.findController) {
        this.findController.setDocument(null);
      }
      if (this._scriptingManager) {
        this._scriptingManager.setDocument(null);
      }
    }
    this.pdfDocument = pdfDocument;
    if (!pdfDocument) {
      return;
    }
    const isPureXfa = pdfDocument.isPureXfa;
    const pagesCount = pdfDocument.numPages;
    const firstPagePromise = pdfDocument.getPage(1);
    
    const optionalContentConfigPromise = pdfDocument.getOptionalContentConfig();
    const permissionsPromise = this.#enablePermissions
      ? pdfDocument.getPermissions()
      : Promise.resolve();