PDF Clown is an Open Source library for creating and manipulating PDF files in Java 1.5 and .NET 2.0. It is distributed under the GNU General Public License, with the LGPL as alternative license.
It features multiple abstraction layers to satisfy different programming
styles: from the lower level (PDF object model) to the higher (PDF document structure and content streaming).
The project was started in 2006, and has been actively developed since then. Many features have been implemented, but there are many planned improvements. PDF specification 1.6 is partially implemented.
The toolset offers PDF document splitting, merging, watermarking, stamping, etc. and low-level access to every single PDF object inside the internal structure.
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.