My questions and this discussion are wholly concerned with reading and using PDFs online. However, I question the assumption that general users print PDFs. I think it's probable that many are read or skimmed online before their readers make a decision about printing them. Two points are relevant. First, PDF is a widely-used format for commercial electronic books.
Publishers sell PDF books with the explicit expectation that they'll be read online.
Writers' e-book websites recommend PDF because the pages look like print. Second, there's an immense amount of effort going into improving resolution for online reading. We discussed e-book devices and the ClearType format briefly on this list late last year. The problems with PDF seem to fall into two groups, PDF readability and Acrobat Reader usability. There's a also a set of issues to do with the relationship between Web browsers and Acrobat.