I am collecting quite a lot of material in a GitHub wiki. I really like to use the wiki to cooperate with other people and IMHO the platform is really nice, I like it!
So, I would like to keep using the GH wiki to collect stuff, edit, save,etc but I also would like to export the content in order to create a pdf file that we can call "a manual". I would like to generate an updated version of the manual automatically everytime I want just running a couple of scripts, I can not put too much effort on this.
I guess it is possible to export the content somehow and the use pandoc (http://johnmacfarlane.net/pandoc/) to create the pdf maybe adding an index and a style file.
Another interesting idea could be publish a website once a month dumping content directly from the wiki.
I guess other people already did something like this but I did not find anynthing. Any idea?
But... the Github wiki of a GitHub repo is a git repo in itself (introduced in August 2010).
You can clone it, push to it or pull from it.
Each wiki is a Git repository, so you're able to push and pull them like anything else.
Each wiki respects the same permissions as the source repository.
Just add ".wiki" to any repository name in the URL, and you're ready to go.
Or, as noted by htafoya in the comments, replace the .git part of the URL (if present) by .wiki.
That makes the "export" part of your question really trivial.
From there, you will find tons of script for converting markdown pages into pdf: