In my day-to-day work, I find I spend a fair amount of time cross-referencing multiple PDF documents. While the tiling functions (In Acrobat, "Window > Tile") can be quite sufficient in some cases, I find that I prefer the flexibility of running multiple instances of Acrobat itself with my dual monitor setup. This way I can keep several documents open concurrently, side-by-side, and it allows me to customize toolbars and the like based on what I plan to do with each document.
Whilst this was possible with Acrobat 5.X, it was quite unstable in my experience. Acrobat 6.0 on the other hand I have found to be far more reliable. It should also be said that this is just a little trick that I've picked up, and as late-night Infomercial hosts are fond of saying, "Individual results may vary".
For those preferring to view multiple documents through a single instance of Acrobat, it is also possible to rapidly switch between open PDFs in Acrobat by hitting Ctrl-Tab.
Anyway, in order to open your multiple instances of Acrobat in windows:
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.