This kind of thing can happen because Word uses the active printer driver to optimise the page layout. Even when the same printer is used with different drivers (as must be the case when one uses a Mac; the other a PC), this can happen. Typically, the changes might mean the odd line ending up on a different page, but this can get messy when a novice does silly things like inserting empty paragraphs to force content to the next page - you might end up with pages containing just a line or two of text. Graphics will likewise move according to changes in their anchor positions and that can lead to objects overlapping. Short of sending PDFs, about the best you can do is ensure both users view the document using drivers for the same printer (the printer doesn't need to be available - provided you don't need to print).

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]