Word Counter is a great online utility which you can use to find the keywords which are used most frequently in a given document.
Simply create an account and the put the text of the document you want to analyze in Word Counter.
The utility will not only detect the total word and character counts, but it will also generate a list of which words are used most frequently.
You can change the keyword density to only include two word or three word phrases.
My friend Nikolai of HashTag Legal demonstrates in YouTube video, how the analysis performed by Word Counter can be implemented in Relativity. The application he has has designed makes keyword density analysis far more useful.
Keyword Density is shown in a separate field, which gives a count for each string.
Hash Tag Legal has made numerous useful tools available here. I highly recommend checking out Nick's site. The general spirit of this site is similar to Litigation Support Tip of the Night.