Starting Word 2007, the program offers the possibility of saving to PDF through an add-in. You just have to choose Save As > PDF.
Install the package below, then restart Word and use Office Button > Save As.
Download package: https://www.microsoft.com/en-us/download/details.aspx?id=7
If it's not working for you, then download CutePDF Writer and you will have the possibility of saving to PDF from Office BUtton > Print.
Download CutePDF Writer: http://www.cutepdf.com/Products/CutePDF/writer.asp
Note: Make sure you have an active Internet connection available because the CutePDF Writer requires the Ghostscript download.