[[Please note that Github has changed a lot since this question was first asked; instead of \"download page\" read \"new release\".]]
I generate PDF documentation as par
check out for net-github-upload which is available
for perl: http://github.com/typester/net-github-upload-perl and ruby: http://github.com/Constellation/ruby-net-github-upload
With that you can write a small script to upload and update your PDF easily. To sad there's no easy way provided by github guys themselves..
cheer!