string path= "aa.pdf"; //Or URL 

            System.Net.WebClient w = new WebClient();

            byte[] f= w.DownloadData(path);

    //保存到数据varbinary字段类型中

            //取出后

            Response.AddHeader("Content-Disposition", "attachment;filename=d.pdf"); 

            Response.ContentType = "Application/pdf";

            Response.BinaryWrite(f);