Response.Buffer = false; //new
    Response.Clear(); //new
    Response.ClearContent();
    Response.ClearHeaders();
    Response.AddHeader("Content-Disposition", "attachment; filename=" + filename);
    Response.ContentType = "Application/pdf";
    try
    {
        Response.TransmitFile(filename);
        Response.End();
    }
    catch
    {
        Response.ClearContent();
    }

ScriptManager.GetCurrent(this.Page).RegisterPostBackControl(Button);