How do I prevent iWork apps from restoring windows from the previous session?
Judging from Apple Discussion threads (a quick search turned up these three threads on the first page of results alone), the global Resume switch does not seem to consistently stop applications from saving and restoring Saved State information.

If you only want to disable Resume selectively for iWork, you may be best served by something like RestoreMeNot or TinkerTool, which has a dedicated tab for granularly managing Resume besides its other tweaking features.

If you want to completely disable the Resume feature, Mac OS X Hints outlines a procedure to do so. I can’t judge on it, nor recommend it, as it is altogether too hackish for my taste – the editor’s label “proceed at your own risk” applies.