Code
 public class AutoDownloadHandler : IHttpHandler
    {
        #region IHttpHandler Members

        public bool IsReusable
        {
            get { return false; }
        }

        public void ProcessRequest(HttpContext context)
        {
            string filePath =“站点上的文件”；//比如http://192.168.7.11:88/test.pdf
             string user = "站点用户"；
             string passWord = “密码”；
            try
            {
            
                HttpWebRequest httpWebRequest;
                httpWebRequest = (HttpWebRequest)HttpWebRequest.Create(filePath);
                NetworkCredential credential= new NetworkCredential(user, passWord);
                httpWebRequest.Credentials = credential;
                HttpWebResponse httpWebResponse;
                httpWebResponse = (HttpWebResponse)httpWebRequest.GetResponse();
                Stream responseStream = httpWebResponse.GetResponseStream();
                BinaryReader binaryReader = new BinaryReader(responseStream);
                byte[] arraryByte= new byte[responseStream.Length];
                binaryReader.Read(arraryByte, 0, (int)responseStream.Length);

                HttpContext.Current.Response.ContentType = "application/pdf";
                HttpContext.Current.Response.AppendHeader("Content-Disposition", "attachment; filename=" + HttpUtility.UrlEncode(fileName, System.Text.Encoding.UTF8));
                HttpContext.Current.Response.AppendHeader("content-transfer-encoding", "gb2312");
                HttpContext.Current.Response.AppendHeader("Content-Length", arraryByte.Length.ToString());

                HttpContext.Current.Response.BinaryWrite(arraryByte);
                HttpContext.Current.Response.Flush();
                responseStream.Close();
                HttpContext.Current.Response.End();

            }
            catch (Exception ex)
            {
             
            }
        }

        #endregion
    }