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).
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.
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.
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.