I upgraded to 8.1 from Win7 last week. My first experience trying to compare a PDF document with text file in another editor was quite unpleasant (Metro essentially makes apps modal, which is nuts).
Doesn't seem like it should be hard for Microsoft to accomplish. Metro's already supposed to be resolution-independent, so all that's really accomplished by forcing them to be fullscreen on desktop monitors is keeping ease of task switching and information density to an absolute minimum.
http://i.imgur.com/3IwDXVA.png
Or, don't use Reader. The inclusion of a free PDF reader app with your computer purchase doesn't get in the way of using a desktop program instead.
I do love the keyboard extensions brought into play with Windows 7 for positioning windows, windows+left/right to align to exactly half of the current window, keeping hitting right or left over and over again to keep moving across monitors. Windows+up toggle maximize. It's ridiculously quick and simple and you can go from a bunch of maximized apps to comparing two things side by side without ever leaving the keyboard.
I can do this on my mac as well, but I have to install a flaky extension of some sort to make it happen.