Before Windows 8, opening multiple PDFs would result in a window for each one, and you could then alt+tab between open documents.
In Windows 8, they all open to the Reader app, but only one instance is ever open. Is there a way to quickly switch between open documents using Reader?
When you have 2 or more files open in Reader you can press and HOLD right click on a file you are not currently viewing and it will let you open it in a new window.
Voilà, you now have two instances of Reader each with different files. Just do the Windows 8 split screen thingy and the can be side by side for your viewing pleasure.
What you want to do is:
Have a nice day.