I think there's another thread that talks about this. As I recall, the answer may have something to do with the PDF being formatted with colors? Or was it grayscales? Rather than strictly B&W. If you created it, play around with those settings. If you didn't ... read it on a tablet instead?? Or just the PC. Or ignore the missing letters?
edit: Actually, even if you didn't create it, try RE-saving a copy on your PC with those settings changed.