File Open / Download

you can try

 private void DownloadFile(string fname, bool forceDownload)
    {
        string path = MapPath(fname);
        string name = Path.GetFileName(path);
        string ext = Path.GetExtension(path);
        string type = ReturnExtension(ext);//provide the exetention of file

        if (forceDownload)
        {
            Response.AppendHeader("content-disposition",
                "attachment; filename=" + name);
        }
        if (type != "")
            Response.ContentType = type;
        Response.WriteFile(path);
        Response.End();
    }