call serverside Buttonclick from Javascript

Question

Hi,
   In my Registration page I want  to generate PDF file after entering the details.  Now I can generate PDF using iTextSharp. But, my problem here is,in Reg.aspx page itself the PDF generated. I want to generate PDF in blank page. How can i do this ? can anyone help me to do this? Here I mention my code.Now i'm trying this using <a /> tag.
In Reg.aspx :

 <asp:Button ID="btnUpload" runat="server" Text="Generate PDF" OnClick="btnUpload_Click" Visible="false"/>

            <a id="pdfconvert" target="_blank" onclick="generatepdf()" runat="server">Convert to PDF</a>

In Javascript :

 function generatepdf() {

            //var btnpdf = document.getElementById("btnUpload")

            var btnpdf = document.getElementById("<%= btnUpload.ClientID %>");

            btnpdf.click();

            // document.getElementById("btnUpload")

        }

In Reg.aspx.cs

protected void btnUpload_Click(object sender, EventArgs e)

        {

            Document pdfDoc = new Document(PageSize.A4, 10, 10, 10, 10);

            try

            {

                PdfWriter.GetInstance(pdfDoc, System.Web.HttpContext.Current.Response.OutputStream);

                pdfDoc.Open();

                //Set Font Properties for PDF File

                Font fnt = FontFactory.GetFont("Times New Roman", 14);

                PdfPTable PdfTable = new PdfPTable(1);

                //PdfPCell PdfPCell = null;

                //Retrieve content from design side

                string pntype = "Reg.Type  : ";

                string ptype = ddlpatienttype.SelectedItem.Text;

                PdfPCell ptypeval = new PdfPCell(new Phrase(new Chunk(pntype + ptype)));

                PdfTable.AddCell(ptypeval);

                string pnamel = "Name : ";

                string pnamet = "";

                if (rdbtnmr.Checked == true)

                {

                    pnamet = lblsalutmr.Text + " " + txtPFirstName.Text + " " + txtPMiddleName.Text + " " + txtPLastName.Text;

                }

                else if (rdbtnms.Checked == true)

                {

                    pnamet = lblsalutms.Text + " " + txtPFirstName.Text + " " + txtPMiddleName.Text + " " + txtPLastName.Text;

                }

                else

                //if (rdbtnmrs.Checked == true)

                {

                    pnamet = lblsalutmrs.Text + " " + txtPFirstName.Text + " " + txtPMiddleName.Text + " " + txtPLastName.Text;

                }

                //string pnamet = ddlSalutation.SelectedItem.Text + " " + txtPFirstName.Text + " " + txtPMiddleName.Text + " " + txtPLastName.Text;

                PdfPCell pnameval = new PdfPCell(new Phrase(new Chunk(pnamel + pnamet)));

                PdfTable.AddCell(pnameval);

 PdfPCell pggenderval = new PdfPCell(new Phrase(new Chunk(pggenderl + pggenderrad)));

                PdfTable.AddCell(pggenderval);

                pdfDoc.Add(PdfTable);

                pdfDoc.Close();

                Response.ContentType = "application/pdf";

                System.Web.HttpContext.Current.Response.Write(pdfDoc);

                Response.Flush();

                Response.End();

            }

            catch (Exception ex)

            {

                Response.Write(ex.ToString());

            }

        }

In Design the link "Convert to PDF" can't worked.