System.Text.StringBuilder outputBuffer=new System.Text.StringBuilder();
string filename=DateTime.Now.ToString("yyyyMMddhhmmss")+".txt";
outputBuffer.AppendFormat("{0}",savetxt);
System.Web.HttpContext.Current.Response.Clear();
System.Web.HttpContext.Current.Response.AddHeader("Content-Disposition", "attachment; filename="+filename);
//System.Web.HttpContext.Current.Response.AddHeader("Content-Length", size);
System.Web.HttpContext.Current.Response.ContentType = "application/octet-stream";    
System.Web.HttpContext.Current.Response.Charset = "UTF-8";
System.Web.HttpContext.Current.Response.BinaryWrite(System.Text.Encoding.UTF8.GetBytes(outputBuffer.ToString()));
System.Web.HttpContext.Current.Response.End();