When viewing a PDF file on GitHub, there seems to be no way to have the browser simply display the file as opposed to downloading it.
None of these links work, and these are the only ones I'm able to get to (or derive from the behaviour of working with source code):
Best Answer
Use Google Docs viewer with a url like:
e.g.
https://docs.google.com/viewer?url=https://raw.githubusercontent.com/degoes-consulting/lambdaconf-2015/master/speakers/jdegoes/intro-purescript/presentation.pdf