I discovered this little jewel of a feature of the new Preview in Leopard the other day when my girlfriend noticed that the chapter of a book she had scanned for class (she's a grad student/TA) had an inch thick border of black on the margins (which as most people who make photocopies know happens all the time) for whatever reason –– something to do with the way the scanner interprets the distance between the glass and the cover. Well, being an environmentalist at heart, she didn't want to waste any more toner than she had to so, she asked me if there was an easy way to crop it.

Somehow, I just intuitively tried the following and it worked! The cropping isn't some smart feature, however, but rather depends on the fact that each page in the PDF you select (and all pages can be selected) needs the same crop.

So, open up whatever PDF you want to crop in Preview. Hit the Select button in the toolbar and crop the first page as you want all the selected pages to be cropped. The cropping tool works just like any crop tool in your average photo editing software. Then, in the sidebar on the right, select a thumbnail of any page (make sure the sidebar is in thumbnail view and not Table of Contents or Annotations view). Now check to be sure that the crop size is what you want to be applied to all the pages you select, then hit Select All, and go to Tools » Crop or hit Command-K. Now your selection is cropped just the way you want it! It couldn't be easier if you ask me.

My girlfriend's PDF scan looked so clean it was as if it was never scanned at all but rather just converted from some .doc file. Disclosure: I just checked the full list of features of Leopard and this hint is vaguely described as "Batch Image Operations." I suppose that images are much like PDFs, but I was genuinely shocked when Preview so efficiently and cleanly cropped my girlfriend's PDF file.