People read text in different ways. Some people read text word for word, moving along a sentence; others speed read, absorbing multiple words at the same time.
Another way of reading is RSVP (http://www.iis.ee.ic.ac.uk/~o.debruijn/project/rsvp.html).

It would be really nice if evince was able to support different methods of reading just as it can support multiple document formats.

There's an example here:
 http://trevor.smith.name/EST/index.html