Please help me onclick button(PDF export) not fire under modalpop extender panel

<pre>   <center>
       
        <asp:Button ID="btnGetSelected" runat="server" Text="Generate Report" Style=" margin-left:270px" OnClick="btnGetSelected_Click" />
         <asp:CustomValidator ID="CustomValidator1" runat="server" Style=" font-size:x-large" ErrorMessage="Please select at least one record." 
    ClientValidationFunction="Validate" ForeColor="Red"></asp:CustomValidator>
      
        <asp:Button ID="ButtonHdn" runat="server" Text="Button" Style="display: none;"  />
        </center>
          
        <cc1:ModalPopupExtender ID="modalpop1" runat="server" PopupControlID="Panel1" TargetControlID="ButtonHdn"  dynamicservicepath=""
                enabled="True"
                 BackgroundCssClass="modalBackground">
        </cc1:ModalPopupExtender>
        <asp:Panel ID="Panel1" runat="server" CssClass="modalPopup" align="center" style="max-height: 500px; overflow: auto; display:none; ">
               <%--<asp:Button ID="btnClose" runat="server" Text="Close" OnClick="btnClose_Click" />--%>
             <asp:ImageButton ID="ImageBtnclose" runat="server" ToolTip="Close" ImageUrl="~/images/cross.png" OnClick="ImageBtnclose_Click" />
           <%-- <asp:ImageButton ID="ImageBtnprint" runat="server" ToolTip="Print" ImageUrl="~/images/print.jpg" OnClick="ImageBtnprint_Click" />--%>
            <asp:Button ID="Button1" runat="server" Text="PDF Generate" OnClick="Button1_Click1" /> This Button not fire on code behind
          <center>                
              <table id="table1" runat ="server" >
                <tr>
                <td>
                  <asp:Literal ID="Literal1" runat="server"></asp:Literal>
                     </td>
                </tr>
                    
                    </table>
              </center>

     
</asp:Panel>
        </ContentTemplate>
 </asp:UpdatePanel>

</div>
    <asp:UpdateProgress ID="UpdateProgress1" runat="server">
        <ProgressTemplate>
    <div class="modal">
        <div class="center">
            <img alt="" src="images/loadingImg.gif" />
        </div>
    </div>
</ProgressTemplate>
    </asp:UpdateProgress>

protected void Button1_Click1(object sender, EventArgs e)
    {
        //StringWriter sw = new StringWriter();
        //HtmlTextWriter hw = new HtmlTextWriter(sw);
        //this.Literal1.RenderControl(hw);
        StringReader sr = new StringReader(Request.Form[Literal1.UniqueID]);
        Document pdfDoc = new Document(PageSize.A4, 10f, 10f, 10f, 0f);
        PdfWriter writer = PdfWriter.GetInstance(pdfDoc, Response.OutputStream);
        pdfDoc.Open();
        XMLWorkerHelper.GetInstance().ParseXHtml(writer, pdfDoc, sr);
        pdfDoc.Close();
        Response.ContentType = "application/pdf";
        Response.AddHeader("content-disposition", "attachment;filename=HTML.pdf");
        Response.Cache.SetCacheability(HttpCacheability.NoCache);
        Response.Write(pdfDoc);
        Response.End();
        //ScriptManager.RegisterStartupScript(this, this.GetType(), "isActive", "printGrid();", true);
        modalpop1.Show();
    }