The Coherent PDF Command Line Toolkit provide a robust, professional set of tools for manipulating PDF files, including split and merge, linearize, encrypt, scale, rotate, crop, bookmark, stamp, watermark, annotate, attach and much more.
They are competitively priced and come with a comprehensive manual.
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.