There are a number of great free tools to do all this work.
Of course for those with cash, Adobe's tools are the best choice, but pdftk is a fantastic free alternative, if you don't mind a bit of command line typing.
An example of putting a foreground stamp onto every PDF with pdftk:
- Generate a PDF where you would like to apply the stamp to (i.e. the source)
- Create a stamp PDF. This should have the stamp you wish to apply (e.g. "DRAFT") and be a transparent PDF. Microsoft Word will generate transparent PDFs by default, so you can create your stamp in Word.
- Apply the stamp to the source PDF using pdftk, for example:
pdftk in.pdf stamp stamp.pdf output out.pdf