Cropping Pages to Highlight Areas in Acrobat
http://www.planetpdf.com/enterprise/article.asp?ContentID=Cropping%5Fpages%5Fto%5Fhighlight%5Fareas%5Fin%5FAcrobat&gid=7435

Shea, Dan
PlanetPDF
2006

Abstract:

When you receive a PDF that contains, say, an image that is surrounded by text, how do you highlight the image? One way is to export the graphic to an imaging application such as Photoshop, but that involves additional applications and the associated loading times. A great 'quick and dirty' fix here is to use the 'Crop' tool to hide the content that surrounds the image, leaving you with PDF which displays only an image, just the way you wanted it. This tip explains how.