Update March 2015: "PDF Viewing"!
Simply browse to a PDF document and we'll render it in your browser like any other file.
From presentations to papers, we've got you covered.
Many thanks to Mozilla and every contributor to PDF.js.
Original answer (October 2014)
This is simply not (yet?) supported in GitHub.
The best you can do is a greasemonkey extension which would allow you to call a pdf viewer, like the recent pdf.js (a Portable Document Format (PDF) viewer that is built with HTML5.), allowing you to view a pdf entirely online, without any pdf plugin installed.
See that example on jsbin.