Ubuntu Unity's dash brings along the power to implement some quite helpful things to make life easier at your computer - these implementations come in the form of lens that use Unity's search bar, such as the Recoll lens.
With Recoll you can perform full-text search in all indexed files on your system - just choose the appropriate lens in the bottom of dash and enter your subject in the search bar:
To download Recoll, open a terminal and perform the following steps to add the appropriate repository, update the sources and install it:
sudo add-apt-repository ppa:recoll-backports/recoll-1.15-on
sudo apt-get update
sudo apt-get install recoll-lens recoll
To update your index, just open the Recoll application from dash and choose Update Index from File on the top bar.