Error Creating PDF from HTML

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ASP.NET

PDF

itextsharp

Hi Experts,
In .aspx page I have asp.Panel inside the panel there's a repeater generated from DB.
On button click I convert the html to PDF Which work very well.
When I try to do the same on Page_Load it gives me error:

The document has no pages.

Tried to fix this by adding:

Document.Add(new Paragraph("Hello World!"));.

but it gives empty PDF file.
So I tried to run my converting function after page load:

protected override void Render(System.Web.UI.HtmlTextWriter writer)
        {
            base.Render(writer);
            Response.Flush();
            ExportPDF();
        }

It gives error:

Server cannot set content type after HTTP headers have been sent

The converting function:

private void ExportPDF()
        {
            Response.ContentType = "application/pdf";
            Response.AddHeader("content-disposition", "attachment;filename=Invoice.pdf");
            Response.Cache.SetCacheability(HttpCacheability.NoCache);
            StringWriter sw = new StringWriter();
            HtmlTextWriter hw = new HtmlTextWriter(sw);
            pnlContents.RenderControl(hw); //the containing panel
            StringReader sr = new StringReader(sw.ToString());
            Document pdfDoc = new Document(PageSize.A4, 10f, 10f, 10f, 0f);

            HTMLWorker htmlparser = new HTMLWorker(pdfDoc);
            PdfWriter.GetInstance(pdfDoc, Response.OutputStream);
            pdfDoc.Open();
            htmlparser.Parse(sr);
            pdfDoc.Close();
            Response.Write(pdfDoc);
            Response.End();
        }

So It looks like something wrong with the asp.Panel.
Please advise why it would work on button click but not on page load or Render. What I am missing?

Thanks for help
Samira