Lion introduces a new tool in Preview. Press ` (the backtick key) or go to Tools » Show Magnifier to launch the magnifier. This produces a floating window that you can drag over your document, and it will zoom in on whatever area of the document is underneath the magnifier.
The size of the magnifier window appears to be fixed, but you can press + or – to change its size. At full zoom, the magnifier tool can end up larger than the size of the document window that you are viewing.
[crarko adds: A few people submitted this hint, and some also pointed out that the pinch-to-zoom gesture works with this feature.]
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Mac OS X Hints
http://hints.macworld.com/article.php?story=20110809230348703