One of the features in the OS X Finder is Quick Look, which lets you bring up an information window for the document, and if the system has a supported plug-in for it, enables you to preview its contents before launching a program to handle it.
If you have more than one item selected in the Finder when using Quick Look, you can use arrow keys to navigate through the selection. If only one item is selected, then pressing the arrow keys with the preview window open will move to the next item in the current Finder folder and display Quick Look information on it.
To open a Quick Look window, you can right-click a document and choose Quick Look, or you can do so from the system's File menu, where you can see that yet another option is to use Command-Y.
A more common approach is to simply tap the spacebar on your keyboard; the system will invoke Quick Look on the selected items. However, if you are using Apple's Magic Trackpad or the built-in trackpad for a MacBook, you can use a hidden gesture to invoke Quick Look. Simply use three fingers to tap the trackpad, and the current item under the mouse cursor will be targeted for Quick Look.
Unfortunately this gesture does not invoke Quick Look on a selection, and if you have items selected, it will deselect all but the item under the mouse cursor. Nonetheless, it may still come in handy as an option to use in addition to the spacebar, command hot key, and menu options.