Fonts change when I open a document that was created in another application

Cause: Documents that are created in other word-processing applications can contain fonts that are not installed on your computer. If you open a document that contains a font that is not installed on your computer, Word substitutes an available font.

Solution: Use fonts that are common on most computers.
Solution: Open Font Book and make sure that the font isn't disabled.
Solution: Install the missing font.
Solution: Specify which font Word uses for substitution.