overlay one pdf or ps file on top of another

Question

I have two pdf or postscript files (I can work with either one). What I want to do is merge each page on top of the other so that page1 of document A will be combined with

Answer 1

For OS X there is PDF letterhead. Doesn't do anything else than just overlaying PDF's. https://itunes.apple.com/us/app/pdf-letterhead/id976548033?mt=12