You can add a watermark to PDF documents by using a special merge field. Here are the specifications.
Depending upon the length and complexity of the Watermark, this functionality may increase the file size of the output file by up to 50%.
A total of three lines of text are available in a PDF Watermark. To add a line break to your Watermark, use the web encoded syntax for a line break -- "%0D%0A"
Value must be between -90 and 90.
Value must be between 1 (fully transparent) and 100 (opaque).
Value can be a hex value or a color from this KnownColor Enumeration list.