If you are working on a project, needing some feature to generate PDF format of invoices or other documents in Java. Several recommendations:
iText is a really good Java / PDF API, iText is a good option to generate PDF with Java. But you have to be very careful while creating your PDFs since you do most of the filling and layout by hand.. http://itextpdf.com/
BIRT: Also many features and provides especially good integration into an Eclipse based environment.