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 dependant; with iText and PDF you can control exactly how your servlet's output will look.
iText requires JDK 1.2. It's available for free under the LGPLicense.
OK, so you want to stamp your document. Maybe you need to give reviewers some advice about the document's status or sensitivity. This tip from author Ted Padova demonstrates how to add stamps with the Stamp Tool along with related comments.