public class PdfPrintDocumentEx : PdfPrintDocument
    {
        public PdfPrintDocumentEx(PdfDocument Document, int mode = 0) : base(Document, mode)
        {
        }

        //Draw watermark in the background (under page content)
        protected override void OnPrintPage(PrintPageEventArgs e)
        {
            //Draw watermark
            e.Graphics.FillRectangle(Brushes.Red, 0, 0, 500, 500);
            //Draw page content
            base.OnPrintPage(e);
        }

        //Draw watermark above page content (with transparency)
        protected override void OnPrintPage(PrintPageEventArgs e)
        {
            //Draw page content
            base.OnPrintPage(e);
            //Draw watermark
            e.Graphics.FillRectangle(new SolidBrush(Color.FromArgb(50, 255, 0, 0)), 0, 0, 500, 500);
        }
    }

     var doc = PdfDocument.Load(@"D:/sample.pdf");

     var pd = new PdfPrintDocumentEx(doc);
     pd.Print();