Word repaginates the document according to the printer driver currently selected. Different printers have different capabilities (eg how close to the edge of the paper they can print) and display content differently. Even on the same computer, changing the printer can result in changes to pagination.

Because your document is being displayed on Windows and Mac it is a safe assumption that two different printer drivers are associated with the docs.

In addition, the typefaces on both machines will also differ although the line breaks in your posted screenshots appear common to both machines.

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Andrew Lockton
Chrysalis Design, Melbourne Australia