Another alternative would be JasperReports: JasperReports Library. It uses iText itself and is more than a PDF library you asked for, but if it fits your needs I'd go for it.
Simply put, it allows you to design reports that can be filled during runtime. If you use a custom datasource, you might be able to integrate JasperReports easily into the existing system. It would save you the whole layouting troubles, e.g. when invoices span over more sites where each side should have a footer and so on.