Not my area of expertise, but I have a brainstorming idea to check on:
Maybe those characters are not in the font the PDF renderer is using. If it's using Advanced PDF/HTML Templates, you can tell it in the XML to use a font that you're sure includes that symbol.
First thing to do before trying to solve the problem though is to identify what exactly the problem is. This is just one possible idea of what the problem is. It could be something else. You can determine if this is the problem by adding those characters into a plain text field that will be printed on the PDF, and see if it has the same problem.