第1步是从您的base64字符串转换为字节数组：

Step 1 is converting from your base64 string to a byte array:

byte[] bytes = Convert.FromBase64String(base64BinaryStr);

第二步是保存字节数组到磁盘：

Step 2 is saving the byte array to disk:

System.IO.FileStream stream = 
    new FileStream(@"C:\file.pdf", FileMode.CreateNew);
System.IO.BinaryWriter writer = 
    new BinaryWriter(stream);
writer.Write(bytes, 0, bytes.Length);
writer.Close();