http://help.adobe.com/en_US/Acrobat/9.0/Professional/WS546948FF-6085-4b14-8640-D9EDE30AD8CB.w.html
Crop a page with the Crop tool
1.Choose Tools > Advanced Editing > Crop Tool.
2.Drag a rectangle on the page you want to crop. If necessary, drag the corner handles of the cropping rectangle until the page is the size you want.
3.Double-click inside the cropping rectangle or shift+return.
The Crop Pages dialog box opens, indicating the margin measurements of the cropping rectangle and the page to be cropped. You can override these settings or apply other options by making new selections in the dialog box before clicking OK.
Crop pages for PDF