If you wish to view files in an Open window, you can use Quick Look. Just select a file and press the space bar to view it. (This works only with those files that Quick View can display.) You can also move around and view other files using the arrow keys.
[kirkmc adds: I had assumed this would already have been on the site, but I only found this hint which offers a workaround. So I wonder when this was added to OS X.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20130325172928724