exporting datagrid to pdf

Question

hi am trying to export datagrid to pdf file by using this piece of code:
string attachment = "attachment; filename=Contacts.pdf";
Response.ClearContent();
Response.AddHeader("content-disposition", attachment);
Response.ContentType = "application/pdf";
StringWriter sw = new StringWriter();
HtmlTextWriter htw = new HtmlTextWriter(sw);
GridView1.RenderControl(htw);
Response.Write(sw.ToString());
Response.End();
the file save at disk very fine but when i open the file it give me the error as the file cannot be opened either it is damaged or some file sent via email..
plz advise
thanks in advance