Did a quick test using K2 2.2 + Joomla 1.5.15 (waiting for 1.5.18 next week) + Phoca 1.0.5
1) Phoca overtakes the PDF function once installed and does have some nice layout features.
2) K2 doesn't handle by default a PDF button as Joomla does (however the print functionality works even better then default's Joomla must say).
After looking in K2 forum for some info, there are users asking the same (pdf) and others that found "a way of doing it" (http://community.getk2.org/forum/topics/get-article-id
) by adding an extra button, but doesn't look (at first sight) as a sustainable and tested method in the long run.
So imho, what I thought is right: there needs to be a link between K2 and Phoca, which by all means is more then a few lines of code in order to do it.
All that is something that might be done in future but is only a part of the issue. The other issue is while Phoca does the job right being very flexible it is not a PDF maker stricto senso (which is normal, it's just a pdf converter for the web) and the articles need to be "re-worked" not only to fit the page but also to have specific layouts (1 image instead of three for example, etc.).
Voilà that's were I stand today, so from my part and what I've seen I'll stick to the caveman-by-hand solution per article as automated is out of my league atm.
Thought (or proven/tested solutions)?