All PDFs, Docs, PPTs etc submitted on HN have the [scribd] link by default[1]. It's a way for people to view the document on their browser without having to download it first (there are still browsers out there like Firefox on OS X that do not have an in built PDF viewer)

[1] - http://www.scribd.com/doc/50406482/Hacker-News-Integrates-Wi...