I had this exact same problem!

The fix, for me at least, was to resize the window. I couldn't believe it at first, either, but after doing it to several frozen PDFs, it looks like it works.

This is for Chropera 26.0.1656.60 and (Cr)A(p)dobe Reader 11.0 on my system, your mileage may vary.

EDIT: Also try minimizing and then bringing it back, that seems to work too.