Some PDFs created by MS Word 2007's Save-as-PDF feature (on Windows) don't display correctly on Mac OS / iOS
There isn't anything you do to Word to improve its PDF output. The best way to guarantee solid cross-platform PDFs is to get Adobe Acrobat (the full version, not Reader) and Distiller and make PDFs through those applications. Unless something has changed since the last time I did this on Windows, you'll output to Postscript through the Print window and create the PDF with Distiller, but you need Acrobat to get the drivers and converters. PDF is an open ISO standard, but Adobe's products are still the best way to go here.