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();
}