PDFsharp & MigraDoc Foundation

  XFont font = new XFont(fontFamilyName, FontSize, XFontStyleEx.Regular); //"Times New Roman" ,22 is fontsize
 PdfDocument doc = PdfReader.Open(sourceFileName, PdfDocumentOpenMode.Modify);
  XBrush brush = new XSolidBrush(XColor.FromArgb(128, 127, 127, 127));  //50%
  foreach (PdfPage page in doc.Pages)
   {
                     using (XGraphics graphics = XGraphics.FromPdfPage(page, XGraphicsPdfPageOptions.Append))
                    {
                        double dwPageWidthInPoints = mediabox.Width;
                        double dwpageHeightInPoints = mediabox.Height;

                        // Convert dimensions to inches
                        double dwpageWidthInInches = dwPageWidthInPoints / 72;
                        double dwpageHeightInInches = dwpageHeightInPoints / 72;
                        double centerX = dwPageWidthInPoints / 2;
                        double centerY = dwpageHeightInPoints / 2;
        graphics.DrawString("CONFIDENTIAL",, font, brush, new XPoint(centerX, centerY));
                     }