hello,
theck out this way :
//create a new instance of pdf document and load an exsiting pdf file
PdfDocument doc = new PdfDocument();
doc.LoadFromFile(@"..\sample.pdf");
//draw footer image
PdfImage footerImage = PdfImage.FromFile(@"..\watermark_bg.JPG");
y = doc.Pages[0].Size.Height - footerImage.PhysicalDimension.Height;
PointF footerLocation = new PointF(margin.Left, y);
doc.Pages[0].Canvas.SetTransparency(0.5f);
doc.Pages[0].Canvas.DrawImage(footerImage, footerLocation);
More details about this method refer - http://www.e-iceblue.com/Tutorials/Spire.PDF/Spire.PDF-Program-Guide/PDF-Footer-Add-PDF-Footer-in-C-VB.NET.html