Creating watermark text on PDF export

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Gridview

PDF

C#5.0

export

, +

Hi, I have this PDF export method. I wanted to add watermarking text as the background. Anyone can help me? Thank you.

private void ExportGridToPDF()
               {

                   username = (string)(Session["username"]);
                   Response.ContentType = "application/pdf";
                   Response.AddHeader("content-disposition", "attachment;filename=" +
                                   username + "'s Purchase.pdf");
                   Response.Cache.SetCacheability(HttpCacheability.NoCache);
                   StringWriter sw = new StringWriter();
                   HtmlTextWriter hw = new HtmlTextWriter(sw);
                   //gv_CartProduct.DataBind();


                   gv_CartProduct.RenderControl(hw);
                   //gv_CartProduct.HeaderRow.Style.Add("width", "15%");
                   //gv_CartProduct.HeaderRow.Style.Add("font-size", "10px");
                   //gv_CartProduct.Style.Add("text-decoration", "none");
                   //gv_CartProduct.Style.Add("font-family", "Arial, Helvetica, sans-serif;");
                   //gv_CartProduct.Style.Add("font-size", "8px");


                   StringReader sr = new StringReader(sw.ToString());
                   string str = "<h1 title=’Header’ align=’Center’> Thank You "
                                  +  username + ", for shopping at
                                   ShopNow</h1><br><table
                                    align=’Center’><tr><td
                                    style=’width:100px;color:green’> You have made
                                    successful payment at ShopNow online shop. Please
                                    keep this invoice. Thank
                                   You</td></tr></table>";
                   StringReader sr1 = new StringReader(str);
                   Document pdfDoc = new Document(PageSize.A4, 7f, 7f, 7f, 0f);
                   HTMLWorker htmlparser = new HTMLWorker(pdfDoc);
                   PdfWriter.GetInstance(pdfDoc, Response.OutputStream);


                   pdfDoc.Open();
                   htmlparser.Parse(sr1);
                   htmlparser.Parse(sr);
                   ConfirmPurchase();


                   pdfDoc.Close();
                   Response.Write(pdfDoc);



               }