What causes all iWork apps to have blank, unusable documents after a reinstall?
I had a little trouble getting iWork '09 to work correctly on Mountain Lion.

When I tried to create a new document from template chooser, it was blank, and I could not do anything.

When I tried to open an existing file, it said it "file  could not be opened".

This applied for Numbers, Pages, and Keynote.

This is how I solved it,


Moved the iWork Folder in Applications to Trash.
Removed /Library/Application Support/iWork '09
Re-Installed iWork (I had installation Media).
Then installed the Mountain Lion iWork updates.


Hope that works for you.