I have not seen random changes to a layout.
Save as PDF uses the current Page Setup to render the PDF file. You may find that it isn't the fields that have moved, but the actual Page Setup. Go into Page Setup and make sure you have the appropriate page setup. If the layout was created using "US Letter", but the Page Setup has changed to "US Letter Small", this could cause the fields to scroll off the right edge.
Another possibility is that the fields use a font that was installed on one machine, but doesn't exist on a second machine, and there is some font substitution, which should not make the field larger, but it could make the print smaller or larger.
Please keep me posted if you discover the cause, or if you can duplicate the problem, please let me know the exact steps.