Word repaginates using the printer metrics of the currently selected printer. If you open the print dialog and change printers then it has to start all over again and this may change the page count as you change printer drivers. There is very little you can do to stop that. You might get some mileage in fiddling with the margin settings so the variability always falls within a 'line height' instead of toggling between x and x+1 lines.

Are there variations in fonts in your document? For instance, if you have bullets in your document, they are likely to be a different typeface and could be making tiny spacing changes that you aren't noticing.

If you've done everything else to control this with paragraph spacing, paragraph pagination settings and line heights then the last thing you can do to 'fake it' is to change the section page setup to vertically justify the page content. This will make tiny adjustments to the line heights to always align the top and bottom of the page. If there are lots of lines on every page, these adjustments are not likely to be noticed.

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Andrew Lockton
Chrysalis Design, Melbourne Australia