iText is a library that allows you to generate PDF files on the fly.

The iText classes are very useful for people who need to generate read-only,
platform independent documents containing text, lists, tables and images.
The library is especially useful in combination with Java(TM) technology-based
Servlets: The look and feel of HTML is browser dependent; with iText and PDF
you can control exactly how your servlet's output will look.

WWW: http://www.lowagie.com/iText/