If I'm logged on to wikipedia.org I see an option PDF Version. I don't see this option at mediawiki.org or if I logged off from wikipedia.org.
Is there an extension implementing this feature?
The same extension (Collections from Pediapress) is being used on the OpenOffice.org Wiki. You can generate output in several formats... PDF, ODT. DocBook etc. (depends on the config what output formats are available). It's quite useful.
C.