PDF-letters showing as question marks

I have not been able to find an answer to this strange problem. I have a Macbook Air (OS Lion) and occasionally when I open a PDF attachment on an email all of what is supposed to be letters and words is a series of boxes with question marks inside. I'm confused because my business partner has the same computer and when we are both sent the same email with the same attachment hers opens just fine. Anyone know how to fix this? Thank you

That sounds like a font problem. When a font that's used in an email attachment that you don't have on your machine, or is missing, that problem will occur. PDF documents are normally just word processor documents that have been converted to PDF for universal viewing.