Well, a forum is a great place for a conversation or discussion.
Post a question, get an answer.
But it's sometimes frustrating to need to read everything to find the tidbit you're looking for.

It's also frustrating trying to organize and format a forum to provide a more focussed presentation.
Personally I find it frustrating that things sort of get lost in the forums.

So... Once the questions have been asked and the solutions discussed on the forum, the resulting understanding can be cut and paste into a wiki which might make it easier to navigate. Again IMHO a hyperlinked wiki is a lot easier to use than a linear word document or PDF (but you can export the contents of the wiki and put it into a PDF if you like...)

The wiki sort of grows "organically", and it is easier to go back and tweak the content and the structure than can be done on a forum.

Of course "your mileage may vary" so choose what works for you.

A good example of a very successful wiki is Wikipedia
FWIW, Note to self: I'll need to go and make some updates to the Wikipedia entry for Ecco-Pro and add a link here...