Macintosh: Library > Application Support > Adobe > Adobe PDF > Settings
Windows: Documents and Settings > All Users > Application Data > Adobe > Adobe PDF > Settings
But, when you save your own customized PDF presets, they are stored in your local user location:
Macintosh: Users > [yourname] > Library > Application Support > Adobe > Adobe PDF > Settings
Window: Documents and Settings > [yourname] > Application Data > Adobe > Adobe PDF > Settings"