you need the whole lot, I think, but Norman can give the definitive answer.
I've never tried, but I don't think you can you use it on its own. Seam-pdf registers a document store as a seam components and needs a conversation scope to efficiently store the documents. I can certainly foresee even more use of Seam features in the future.
That being said, except for the need for conversation scope, it isn't unreasonable to think that it could be refactored a bit so as not to require Seam. That isn't a priority of mine, but given sufficient demand and/or community contribution it is entirely possible.