protected void GetFile(object sender, EventArgs e)
{
   string fileName = "Contratto_Licenza_Dinamico_Educational.pdf";
   string script = string.Format("window.location.href='{0}DownloadHanlder.ashx?url={1}'",this.RootUrl, fileName);
   this.ClientScript.RegisterStartupScript(this.GetType(), this.GetType().Name, script, true);
}

public class DownloadHanlder : IHttpHandler
    {

        public void ProcessRequest(HttpContext context)
        {
            string url = context.Request.QueryString.Get("url");
            string path = context.Server.MapPath(url);
            string fileName = Path.GetFileName(path);

            HttpResponse response = context.Response;
            response.Clear();
            response.ContentType = "application/pdf";
            response.AppendHeader("Content-Disposition", string.Format("attachment;filename={0}", fileName));
            HttpContext.Current.Response.TransmitFile(path);
            response.Flush();
            response.End();
        }

        public bool IsReusable
        {
            get
            {
                return false;
            }
        }
    }