There is no way Word can automatically reformat documents to accommodate the differences in A4 & Letter layouts. What is more, Word doesn't change them. If someone sends you a document formatted for Letter stationery, that's how it will be opened and displayed on your system - not in an A4 format. That said, for display and printing purposes, you might want to check Word's option: 'Scale content for A4 or 8.5 x 11" paper sizes'.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]