将文件以流的形式另存为下载

//【1】附件为本地磁盘路径（附件可以为图片、pdf、word等等）
string strFile = Server.MapPath("/images/526763.pdf");
using (FileStream fs = new FileStream(strFile, FileMode.Open))
{
    byte[] bytes = new byte[(int)fs.Length];
    fs.Read(bytes, 0, bytes.Length);
    fs.Close();
    Response.ContentType = "application/octet-stream";
    Response.AddHeader("Content-Disposition", "attachment; filename=" + HttpUtility.UrlEncode("base.pdf", System.Text.Encoding.UTF8));
    Response.BinaryWrite(bytes);
    Response.Flush();
    Response.End();
}

//【2】附件为网络路径（附件可以为图片、pdf、word等等）

WebClient wc = new WebClient();
wc.Credentials = CredentialCache.DefaultCredentials;
byte[] btPageData = wc.DownloadData("http://www.baidu.com/img/bd_logo1.png"); //获取网络附件流
Response.ContentType = "application/octet-stream";
Response.AddHeader("Content-Disposition", "attachment; filename=" + HttpUtility.UrlEncode("base.png", System.Text.Encoding.UTF8));
Response.BinaryWrite(btPageData);
Response.Flush();
Response.End();
string strTargetHtml = Encoding.Default.GetString(btPageData);
wc.Dispose();