That really depends on what's causing the gaps. Word's paragraph Widow/Orphan control, for example, prevents just a single line of a paragraph appearing on a page. That typically results in the last line on a page being moved to the next page. Similarly, if your paragraphs are formatted with a non-zero space before/after that will inevitably result in uneven spacing when a new paragraph starts and there is insufficient room remaining on the page to accommodate even one line (plus spacing) of the new paragraph. Furthermore, if any of your content uses different fonts and/or point sizes, that too will change how many lines will fit on a page.

Finally, be aware that Word uses the current printer driver for document layout. Changing printers - as is likely to happen if the document is viewed on a different computer - may also change how many lines of text will fit on a given page.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]