您需要dart:io将文件保存到自定义路径。请注意，您还需要：

• 提供一个正确的路径来保存文件。你可以使用path_provider包。
• 允许存储权限。您可以使用permission_handler包来实现此目的。

看看下面的示例应用程序，它完全符合你的需要。_savePdf()函数是神奇的地方。注意，你还需要遵循permission_handler的一些设置步骤（AndroidManifest.xml，build.gradle，...），这些都在指南中。

import 'dart:io';

import 'package:flutter/material.dart';
import 'package:path_provider/path_provider.dart';
import 'package:pdf/widgets.dart' as pw;
import 'package:permission_handler/permission_handler.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  Future<void> _savePdf() async {
    // 1. Create a working directory where you can save the file
    Directory? appDocDirectory = await getExternalStorageDirectory();

    // 2. Request permissions to access storage
    await Permission.storage.request();

    // 3. Just like you did, create the document and add your pages
    final pdf = pw.Document();
    pdf.addPage(
      pw.Page(
        build: (pw.Context context) => pw.Center(
          child: pw.Text('Hello World!'),
        ),
      ),
    );

    // 4. Set the full path where you want to save the file
    final file = File('${appDocDirectory!.path}/example.pdf');

    // 5. Write the file
    await file.writeAsBytes(await pdf.save());
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _savePdf,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}