I've seen this pop up on three different user's macs now. They go to open a PDF and acrobat opens but no window shows up. If you look real close you can see a tiny little bar of pixels on the edge of the screen somewhere and find that the window is usually collapsed to just the bar and pushed off screen. It shrinks smaller than all the other application windows are able to.
Any thoughts on what is causing this or any work arounds?
Use Window > Tile > Horizontal