You may also know about Quick Look, the built in feature that allows you to, well, take a quick look at a document or image.
A little tip for these 2 great apps is to use it with multiple files at one time.
For Quick Look, select your multiple files within Finder, and simple press Spacebar.
The documents open up, but you only see one of them.
Find the ‘icon-view’ button on the toolbar, to then see all the items as thumbnails.
From here you can select any of the items, and it will open up as the primary window in Quick Look.
And of course from here, you can select the “Open with Preview” button on the toolbar too.
You can even Share them in Mountain Lion with Air Drop, Email and Messages.
But if you prefer to use Preview, then it also has a simple feature for multiple files.
Once again, select the necessary files in Finder.
You won’t be able to press Return, but you will be able to control-click (right-click) and select Open with Preview, or simply Open.
The Menu Bar of Preview gives it away, but if you don’t see all the documents, change the view to be Thumbnails to see all your files on the left.
Simple little tip, but can be effective if you like to work with multiple files at the same time.