Try the given below code -
WebClient req = new WebClient();
HttpResponse response = HttpContext.Current.Response;
response.Clear();
response.ClearContent();
response.ClearHeaders();
response.Buffer = true;
response.AddHeader("Content-Disposition", "attachment;filename=\"" + System.IO.Path.GetFileName(HttpContext.Current.Server.MapPath(FileName)) + "\"");
byte[] data = req.DownloadData(HttpContext.Current.Server.MapPath(FileName));
response.BinaryWrite(data);
response.End();