If fonts are not displayed as expected when you open documents or you cannot open documents, it's possible fonts have been corrupted. You can validate fonts to confirm they are still safe to use, and remove any corrupted fonts you find.

Take care not to remove system fonts, located in /System/Library/Fonts; removing system fonts could cause some applications to quit unexpectedly.

To validate fonts:

You can validate font files before installing the fonts. To do so, in Font Book choose File > Validate Files. Browse to and select a font file to validate and click Open. Font Book displays the Font Validation window and indicates whether it found problems with the font.