When the decision letters pdf throws an error and won't open, it usually means that something wasn't completed on one of the records being printed. Below are the two issues we've ran into so far that cause the letters not to export properly.
1. Decisions not confirmed and/or released. Every decision must be confirmed and released for it to be printed properly.
2. Decision Letters not attached to decisions. Every decision must have a decision letter attached to it. Leaving the "Default for Decision" selected will not work.