If you don't use WordPerfect, you may not have the WPMathA fonts used in some of the PDF documents provided on the web (which may result in various equations being unreadable). You can install the needed fonts from Corel by going to WPFonts.exe , which should solve the problem.
You should extrct the fonts in this file in your Windows font folder (e.g. C:\Windows\Fonts)