How do I uninstall Adobe Reader 9 for Mac?

You should be able to just delete the application and its associated files. This thread has a list of things and discussion of what to delete. Make sure to remove /Library/Internet Plug-Ins/AdobePDFViewer.plugin if you've installed the Safari in-browser viewer from Adobe.

For most applications, OS X doesn't have installers, or uninstallers, per se. Usually, you simply drag the application Icon to the Trash Can. The app icon (Something.app) is a whole directory. However, this doesn't work for every program.

To remove all traces of a program, if it littered the OS with other files, you might need a program like AppCleaner.app or you can follow this guide for more information.