The retina iPads are among the finest PDF reading devices in existence at present. Combine with a good PDF markup app (iAnnotate or GoodReader, for example), and you've got a nice document-handling workflow that lets you highlight, annotate, and full-text search.
So if that's your imagined use case, it's an excellent solution for that case. Just be sure to get a retina version, as the PDF experience is noticeably improved.
That being said, you do seem well-supplied with devices of various form factors already. So the counterpoint to consider is: how much is a first-class PDF-reading experience worth to you, when you already have access to a business-class experience of the same thing?