You often find crackers put malware into those sorts of files. Usually they use windows and bind them together with 'IExpress' to make the malware execute at the same time as the pdf is opened. It is advised to give your system a quick scan incase they used blackshades (malware that runs on mac as well as windows).
In future open files with a text editor like vim or nano when you are in doubt (if it is a formatted document such as pdf or a document file). You can sometimes tell if there are unusual characters in it.