以字节流的形式下载PDF，然后在Xamarin.Forms中打开默认的安卓应用程序

using System;
using System.Net.Http;
using Xamarin.Forms;

namespace YourApp
{
    public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            InitializeComponent();
        }

        private async void DownloadAndOpenPDF()
        {
            try
            {
                HttpClient httpClient = new HttpClient();
                byte[] pdfBytes = await httpClient.GetByteArrayAsync("http://example.com/path/to/pdf");

                // Save the PDF file locally
                string filePath = DependencyService.Get<ISaveFile>().SaveFile(pdfBytes, "example.pdf");

                // Open the PDF file using the default Android app
                DependencyService.Get<IOpenFile>().OpenFile(filePath);
            }
            catch (Exception ex)
            {
                // Handle any exceptions
            }
        }
    }
}

using Xamarin.Forms;

[assembly: Dependency(typeof(SaveFileImplementation))]
namespace YourApp.Droid
{
    public class SaveFileImplementation : ISaveFile
    {
        public string SaveFile(byte[] fileBytes, string fileName)
        {
            string filePath = Path.Combine(Android.OS.Environment.ExternalStorageDirectory.AbsolutePath, fileName);
            File.WriteAllBytes(filePath, fileBytes);
            return filePath;
        }
    }

    public class OpenFileImplementation : IOpenFile
    {
        public void OpenFile(string filePath)
        {
            Java.IO.File file = new Java.IO.File(filePath);
            file.SetReadable(true);
            string mimeType = MimeTypeMap.Singleton.GetMimeTypeFromExtension(MimeTypeMap.GetFileExtensionFromUrl(filePath));
            Intent intent = new Intent(Intent.ActionView);
            intent.SetDataAndType(Android.Net.Uri.FromFile(file), mimeType);
            intent.SetFlags(ActivityFlags.ClearTop);
            intent.SetFlags(ActivityFlags.NewTask);
            Forms.Context.StartActivity(intent);
        }
    }
}

namespace YourApp
{
    public interface ISaveFile
    {
        string SaveFile(byte[] fileBytes, string fileName);
    }

    public interface IOpenFile
    {
        void OpenFile(string filePath);
    }
}