Spotlight, Mac OS X search engine kinda different with other OS like windows. It make an index or database of file and its content in in your hard drive, and later if you want to find something it will look at the database.

So, after you have new file like *.pdf, it will started to use your CPU so much to do the indexing process (solution: ATSServer and PDF). The same process happened when you finish an installation of Apple software, like Mac update, iLife, or Developer Tools. The CPU ‘eater’ process name is mds. For this problem, you have to add your drive volume to make it stop. But like the ATSServer problem, I keep it running for a while.