I would like to add a Tiled Pavement Watermark as show in the following image.
The simplest type of stamping in PDFNet is done with ‘pdftron.PDF.Stamper’ as shown in Stamper sample ( http://www.pdftron.com/pdfnet/samplecode.html#Stamper), however the Stamper does not support content ‘tiling’ feature that you need. Instead you would need to use ElementBuilder/Writer as described in the following article:
Another approach with ElementBuilder/Writer (which I would recommend) would be to create a pattern (e.g. with text or an image) and then apply this pattern as a fill color to a rectangle. For a concrete sample code, please take a look at Pattern sample: http://www.pdftron.com/pdfnet/samplecode.html#Pattern