Try this
string filepath = Server.MapPath("test.txt");
FileInfo file = new FileInfo(filepath);
if (file.Exists)
{
Response.ClearContent();
Response.AddHeader("Content-Disposition", "attachment; filename=" + file.Name);
Response.AddHeader("Content-Length", file.Length.ToString());
Response.ContentType = "text/plain";
Response.TransmitFile(file.FullName);
Response.End();
}