I also could see the contents of Pages & Numbers files is previews, but they were blank when opened. Apple tech support--in an hour long call--managed to find a fix for my macbook pro. I hope it works for you.
Concurrently to the seemingly blank files, my computer happened to be slowing down & stalling a lot, but while related to the problem, I don't know that this is a necessary symptom of anyone else's.
We deleted all the cache files in the ~/Libary/Caches folder, & emptied the trash. It said some files were in use, so we restarted and finished the trash disposal. Then we started up Pages & Numbers, and it was all good! My computer went back to its normal snappy responses too.
I am sure someone could possibly narrow down which cache files might be causing an issue. But give this a try.
By the way, I am running Pages 9, on Mountain Lion 10.8.4 (recently updated). Disk Utility found preferences and disk verify normally. Activity Monitor showed no excessive CPU use and there was no shortage of hard drive space. The problem was reproducable in safe mode. Deleting plists for iWork had no positive result. Sometimes the computer would get stuck in spinning beach ball mode for a while, or freeze. The only think that worked was the cache purge.
Good luck!