Exporting aspx page to pdf

At last i have exported the UpdatePanel in that aspx page to pdf file instead of exporting the whole page to pdf .If I am writing "this.Page" instead of "UpdatePanel1" it results in an exception Unable to cast object of type 'iTextSharp.text.html.simpleparser.CellWrapper' to type'iTextSharp.text.Paragraph'.

protected void imgbtnprint_Click(object sender, ImageClickEventArgs e)
    {
        Response.Clear();
        Response.Buffer = true;
        Response.ContentType = "application/pdf";
        Response.AddHeader("content-disposition", "attachment;filename=UserDetails.pdf");
        Response.Cache.SetCacheability(HttpCacheability.NoCache);
        StringWriter sw = new StringWriter();
        HtmlTextWriter hw = new HtmlTextWriter(sw);
        UpdatePanel1.DataBind();
        UpdatePanel1.RenderControl(hw);
        StringReader sr = new StringReader(sw.ToString());
        Document pdfDoc = new Document(PageSize.A2, 10f, 10f, 100f, 0f);
        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 txt_salutaion)
    {
        /* Verifies that the control is rendered */
    }