Given that the online documentation is continuously updated, I can refer to that more often and report any issues I encounter. Some time back I started using the online documentation but ran in to trouble along the way and switched back to the pdf versions, but I don't recall what the problem was. It might have been difficulties printing excerpts that I could markup and keep at my desk I'll start relying more on the online documentation.
With respect to how often to update the pdf versions, I'd say as often as practical but not too often. I like having it available for use offline. Like other's I think adding features and correcting problems are more important than maintaining pdf documentation.
Also it's confusing to get a new copy of an old PDF file with each release. I'd prefer that the PDF file not be included in updates unless it's been updated and has a new version number.
Bill