save panel values as pdf file in windows .net application

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C#

I want to save my .Net Windows Panel value in Pdf file,but when save as pdf only one panel value save as one pdf page,another page not open.i want to make more than one pdf page.
this is my code for save pdf file.

Bitmap bmp = new Bitmap(this.panelMain.Width, this.panelMain.Height);
            this.panelMain.DrawToBitmap(bmp, new Rectangle(0, 0, this.panelMain.Width, this.panelMain.Height));

            // Create new pdf document and page
            PdfDocument doc = new PdfDocument();
            PdfPage oPage = new PdfPage();

            // Add the page to the pdf document and add the captured image to it
            doc.Pages.Add(oPage);
            XGraphics xgr = XGraphics.FromPdfPage(oPage);

            xgr.DrawImage(bmp, 0, 0);

            saveFileDialog1.Filter = ("PDF File|*.pdf");
            DialogResult btnSave = saveFileDialog1.ShowDialog();
            if (btnSave.Equals(DialogResult.OK))
            {
                doc.Save(saveFileDialog1.FileName);
                doc.Close();
            }

plz help me