Possibly the default to open PDF is GIMP.
To check and change. In file manager right click on a PDF. Note The application at the top. It should be Document viewer. If it is GIMP that is your problem.
To change to Document viewer Select Open with other application. Select Document viewer from list. Make sure check box to "remember this" at bottom is checked.
ACER Aspire 5750G laptop
Intel i7 - 2630QM 2GHz
Linux Mint 14 (Nadia) 64 bit Mate
Graphics NVidia Optimus
NVidia GeForce GT 540M (1760MB)
Runs using Bumblebee