I don'ty know about pdfs, but you can update your software. Plug in your nook, let it charge all the way, then turn on wifi and let it sit idle. The update should download automatically.
eli62 I doubt any eReader (eInk Ereader) has dictionary working in PDFs. Most of them struggle because they don't really treat PDFs as word by word books. They treat PDFs as images.
I know one of the Kindles added support for highlighting PDFs but it was really broken.
Perhaps updating solves it for you. If it doesn't then the reason probably is that PDFs aren't treated the way other format books are.
Also, some PDFs literally are images i.e. they are collections of images. It might be text on it but it's saved with each page as an image.