exporting gridview data to pdf code given below is not working

using iTextSharp.text;
using System.IO;
using iTextSharp.text.pdf;

using iTextSharp.text.html;

using iTextSharp.text.html.simpleparser;
 Response.ContentType = "application/pdf";

        Response.AddHeader("content-disposition", "attachment;filename=Export.pdf");
        Response.Cache.SetCacheability(HttpCacheability.NoCache);
        StringWriter sw = new StringWriter();
        HtmlTextWriter hw = new HtmlTextWriter(sw);
        HtmlForm frm = new HtmlForm();
        GridView1.Parent.Controls.Add(frm);
        frm.Attributes["runat"] = "server";
        frm.Controls.Add(GridView1);
        frm.RenderControl(hw);
        StringReader sr = new StringReader(sw.ToString());
        Document pdfDoc = new Document(PageSize.A4, 100, 100, 100, 100);
        HTMLWorker htmlparser = new HTMLWorker(pdfDoc);
        PdfWriter.GetInstance(pdfDoc, Response.OutputStream);
        pdfDoc.Open();
        htmlparser.Parse(sr);
        pdfDoc.Close();
        Response.Write(pdfDoc);
        Response.End();


    }
    public override void VerifyRenderingInServerForm(Control control)
    {
        

        /* Verifies that the control is rendered */

    }