solved
class Example extends StatefulWidget {
  const Example({super.key});

  @override
  State<Example> createState() => _ExampleState();
}
class _ExampleState extends State<Example> {

  Future<String> getPdfURL() async {
    await Future.delayed(const Duration(seconds: 5));

    return "http://example.com/pdf.pdf";
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: FutureBuilder(
        future: getPdfURL(), // add your future method here
        builder: (context, snapshot) {
          if (snapshot.connectionState == ConnectionState.waiting) {
            return const CircularProgressIndicator();
          } else if (snapshot.hasError) {
            return Text('Error: ${snapshot.error}');
          } else {
            String url = snapshot.data!;
            return YourPDFViewerWidget(url: url);
          }
        },
      ),
    );
  }
}