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!
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I'm adding to this answer, in case it helps any new readers :) here's what I did:
I installed GitHub Desktop: https://desktop.github.com/
Then, on the wiki page in my repository, I clicked "Clone in Desktop"
This saved the wiki locally as a .md file (after following the steps on screen)
I then used http://www.markdowntopdf.com/ to convert it to pdf (Note: I renamed the files to remove characters that wouldn't work in a pdf file name before uploading to the website)
The end result was really nice.