Problem While exporting gridview data to pdf?

Question

while exporting gridview data to pdf.Header text does not displayed in pdf. Data's only displayed.can anyone help me out...
here is my code....

Html:

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" CellPadding="4"
                        ForeColor="#333333" GridLines="None" Width="773px">
                        <RowStyle BackColor="#E3EAEB" />
                        <FooterStyle BackColor="#1C5E55" Font-Bold="True" ForeColor="White" />
                        <PagerStyle BackColor="#666666" ForeColor="White" HorizontalAlign="Center" />
                        <SelectedRowStyle BackColor="#C5BBAF" Font-Bold="True" ForeColor="#333333" />
                        <HeaderStyle BackColor="#1C5E55" Font-Bold="True" ForeColor="White" />
                        <EditRowStyle BackColor="#7C6F57" />
                        <AlternatingRowStyle BackColor="White" />
                       
                        <Columns>
                        <asp:TemplateField>
                        <ItemTemplate>
                        <asp:Label ID="lbllid" runat="server" Visible="false" Text='<%#Eval("T_ID") %>'></asp:Label>
                        </ItemTemplate>
                        </asp:TemplateField>
                       
                        <asp:TemplateField HeaderText="Serial No">
                        <ItemTemplate>
                        <%#((GridViewRow)Container).RowIndex+1 %>
                        </ItemTemplate>
                        </asp:TemplateField>
                       
                        <asp:TemplateField HeaderText="Branch">
                        <ItemTemplate>
                        <asp:Label ID="iblbranchname" runat="server" Text='<%#Eval("BranchName") %>'></asp:Label>
                        </ItemTemplate>
                        </asp:TemplateField>
                       
                        <asp:TemplateField HeaderText="Type">
                        <ItemTemplate>
                        <asp:Label ID="ibltype" runat="server" Text='<%#Eval("Type") %>'></asp:Label>
                        </ItemTemplate>
                        </asp:TemplateField>
                       
                        <asp:TemplateField HeaderText="Brand">
                        <ItemTemplate>
                        <asp:Label ID="iblbrand" runat="server" Text='<%#Eval("Brand") %>'></asp:Label>
                        </ItemTemplate>
                        </asp:TemplateField>
                       
                        <asp:TemplateField HeaderText="Received Date">
                        <ItemTemplate>
                        <asp:Label ID="iblreceiveddate" runat="server" Text='<%#Eval("ReceivedDate") %>'></asp:Label>
                        </ItemTemplate>
                        </asp:TemplateField>
                       
                        <asp:TemplateField HeaderText="Stock Received">
                        <ItemTemplate>
                        <asp:Label ID="iblstockreceived" runat="server" Text='<%#Eval("ReceivedStock") %>'></asp:Label>
                        </ItemTemplate>
                        </asp:TemplateField>
                       
                        <asp:TemplateField HeaderText="Stock On Hand">
                        <ItemTemplate>
                        <asp:Label ID="iblstockonhand" runat="server" Text='<%#Eval("StockOnHand") %>'></asp:Label>
                        </ItemTemplate>
                        </asp:TemplateField>
                       
                        <asp:TemplateField HeaderText="Sold Date">
                        <ItemTemplate>
                        <asp:Label ID="iblsolddate" runat="server" Text='<%#Eval("SoldDate") %>'></asp:Label>
                        </ItemTemplate>
                        </asp:TemplateField>
                       
                        <asp:TemplateField HeaderText="Quantity">
                        <ItemTemplate>
                        <asp:Label ID="iblquantity" runat="server" Text='<%#Eval("Quantity") %>'></asp:Label>
                        </ItemTemplate>
                        </asp:TemplateField>
                       
                        </Columns>
                    </asp:GridView>


C#;

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;
using System.IO;
using System.Net;
using iTextSharp.text;
using iTextSharp.text.html.simpleparser;
using iTextSharp.text.pdf;

public partial class Admin_SalesReport : System.Web.UI.Page
{
    SqlConnection Con;
    SqlCommand Cmd;
    SqlDataAdapter Sdap;
    protected void Page_Load(object sender, EventArgs e)
    {
        Con = new SqlConnection(@"Data Source=SMAART-1F1D3825\SQLEXPRESS;Initial Catalog=SundarTyres;Integrated Security=True");
        if (!IsPostBack)
        {
            ddl1bind();
            DropDownList2.Items.Add("<--Select-->");
        }
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        gvbind();
        btnprint.Visible = true;
        Btndownload.Visible = true;
    }
    public override void VerifyRenderingInServerForm(Control control)
    {
        /* Verifies that the control is rendered */
    }
    protected void Btndownload_Click(object sender, EventArgs e)
    {
        using (StringWriter sw = new StringWriter())
        {
            using (HtmlTextWriter hw = new HtmlTextWriter(sw))
            {
                //To Export all pages
                GridView1.AllowPaging = false;
                this.gvbind();

                GridView1.RenderControl(hw);
                StringReader sr = new StringReader(sw.ToString());
                Document pdfDoc = new Document(PageSize.A2, 10f, 10f, 10f, 0f);
                HTMLWorker htmlparser = new HTMLWorker(pdfDoc);
                PdfWriter.GetInstance(pdfDoc, Response.OutputStream);
                pdfDoc.Open();
                htmlparser.Parse(sr);
                pdfDoc.Close();

                Response.ContentType = "application/pdf";
                Response.AddHeader("content-disposition", "attachment;filename=GridViewExport.pdf");
                Response.Cache.SetCacheability(HttpCacheability.NoCache);
                Response.Write(pdfDoc);
                Response.End();
            }
        }
    }
    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        DropDownList2.Items.Clear();
        ddl2bind();
    }
    public void ddl1bind()
    {
        try
        {
            Con.Open();
            Cmd = new SqlCommand("Select Distinct Branch From User_Roles", Con);
            SqlDataReader dr;
            dr = Cmd.ExecuteReader();
            DropDownList1.Items.Add("<--Select-->");
            while (dr.Read())
            {
                DropDownList1.Items.Add(dr["Branch"].ToString());
            }
        }
        catch
        {
        }
        finally
        {
            Con.Close();
        }
    }
    public void ddl2bind()
    {
        try
        {
            Con.Open();
            Cmd = new SqlCommand("Select SoldDate From StockManagement Where BranchName='" + DropDownList1.SelectedItem.Text + "'", Con);
            SqlDataReader dr;
            dr = Cmd.ExecuteReader();
            DropDownList2.Items.Add("<--Select-->");
            while (dr.Read())
            {
                DropDownList2.Items.Add(dr["SoldDate"].ToString());
            }
        }
        catch
        {
        }
        finally
        {
            Con.Close();
        }
    }
    public void gvbind()
    {
        try
        {
            Con.Open();
            Sdap = new SqlDataAdapter("Select * From StockManagement Where BranchName='"+DropDownList1.SelectedItem.Text+"' and SoldDate='"+DropDownList2.SelectedItem.Text+"' ", Con);
            DataTable Ds = new DataTable();
            Sdap.Fill(Ds);
            GridView1.DataSource = Ds;
            GridView1.DataBind();
        }
        catch
        {
        }
        finally
        {
            Con.Close();
        }
    }
}