PDFs Can Be Uncropped
Be careful if you are exchanging PDFs that you have cropped with other parties. If you crop a PDF in Adobe Acrobat, even after you save it as a new file, and email it, another user can still open it in using different software.
As the screen grabs below show, it's possible to crop a PDF in Adobe Acrobat, and then undo the cropping action using Nuance Power PDF advanced.