When running a Content Search, search terms are automatically highlighted in the document viewer.  The Keyword Search dialog will display just below the document viewer toolbar to the right.

The dialog displays the index of the current hit and the total number of hits in the top right corner of the panel.  You can move forward to the next hit by pressing F3, and back with Shift + F3.  Alternately, clicking the forward and back arrows in the Keyword Search dialog will move between hits.

To remove highlighted hits, click the X in the top right corner of the dialog.  You can override hits highlighting and perform ad-hoc text searching by pressing Ctrl + F at any time.