There are two library folders in Mac OS X, the one in your Home folder and the other at the root of the hard drive.

The one you need is the one at the root of the hard drive.

Navigate there and to the Internet Plugins folder and you should see a whole bunch of plug-ins.

Preview is the default PDF viewer in OS X but Adobe Reader can also be used. However, this sometimes causes a problem with the two plugins.
I have disabled the Adobe plugin in Safari and use Preview to view PDF documents on the Web. I still have the choice of using Adobe reader for downloaded PDF documents.