If you happen to rebuild a mac and put 10.9.5 on it and you install iWork ’09 from an older CD/DVD so you get iWork 9.0, you will find that you cannot open .docx files.
Going to the AppStore is fruitless – just wants to get you to update to the current versions of Pages (the rewrite) and then tells you that you have an incompatible system (requires OS X 10.10 or later). It doesn’t offer to update you to the updates for iWork ’09 that you may have been offered back when you first installed iWork and before the new iWork apps came out.
The solution is to get the updater from Apple’s Support area (search for “iWork 09 update”). Here’s a link to the newest we found (9.3).