Rather than write up a review of the three main PDF drivers marketed for the AS/400 (which would require that I had actually used all three products), I decided to provide links to them:
Of the three, I have only used Export Ventures' PDFWriter/400. It is a simple, easy-to-use PDF driver for SCS printers. It supports watermarks and JPEG images and has the easiest interface. Simply point your spool file at the PDF driver (output queue), and you get PDF.
The Gumbo PDF Spool-A-Matic has the most features and output options; if you need a lot of output choices, Gumbo may the choice for you. Of course, more choices bring more complexity.
The IBM product is probably the most curious. It works only with IPDS printer data streams.