When I “select text” on my document: text disappears, text partially selectable, text changes to strange symbols etc. Why?

The document may be off spec or you may want to check your system for font errors. To check your system for font errors :

1.  Launch Font Book in your Applications folder.

2.  Select the first item in the Fonts (middle) column, then choose Edit > Select All from the menu followed by File > Validate Fonts. If there are problems, you'll need to remove the problem fonts then quit and re-launch PDFpen again.