Preview app in Mac OS X Lion has a new “Magnifier” tool that allows you to zoom into specific portion of document with a loupe view. This is in addition to the standard zoom in/out which applies to the whole document.
To activate the Magnifier tool, select from menu Tools->Show Magnifier or simply press the backtick ` key.
A loupe view will be displayed once the tool is activated. You can move the loupe and adjust the size of the loupe using pinch gesture.
When you use the Magnifier of Preview app on a PDF document, the loupe changes to a rectangular shape, with its size and position adapts to the document content such as paragraph boundary while you move or pinch to size.