You can install Acrobat Viewer for OS X, but a PDF viewer called Preview that is included with OS X works with PDF’s on OS X. It’s in your Applications Folder. And until you install Acrobat Viewer or Acrobat Pro, PDF’s will automatically open in Preview. Start up Preview and look in the Help menu for more information.
With Preview, you can also combine PDF’s and add pages, unlike Acrobat Reader: Mac OS X v10.6: How to combine PDF documents using Preview.
Safari, your web browser features inline PDF viewing with a built-in PDF engine, so you don’t need an extra viewer, like the Acrobat PDF plugin that is installed with Acrobat reader. If you click on a PDf link in a web page, Safari will open it with the built in viewer.