Compatibility Mode has nothing to do one way or another with whether you can add page numbers - you can insert them regardless of whether the document is in Compatibility Mode.

Without actually seeing the problem document, it can be difficult for anyone to diagnose the issue. Can you attach a document to a post with some representative data (delete anything sensitive)? You do this via the paperclip symbol on the 'Go Advanced' tab at the bottom of this screen.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]