Hi
In Acrobat X, this is how you do:
1. Go to Tools and select the content tab. Then select the 'Edit Object' tool
2. Then select the image that you want to clip (or crop)
3. Hover over the image, and either right click (PC) or ctrl-click mac and choose 'Set clip'
4. Then postion the cursor over one of the corners where you want to start cropping (it needs to be slightly outside of the image), till you see the crop tool appear, not the scissors, then drag the handle in to start cropping the image. And your done.
5. If you want to enlarge the picture afterward since you have made it smaller, its easier if you click of the image and select it again, then position the cursor so its perfectly over a corner point (till 4 arrows becomes just TWO), then click and drag to enlarge.
Bear with it, as it can be quite tricky to get the corners. If you mess up with the clipping, dont worry just try again, you will be greated with an alert that says you have already set the clip, but just carry on and proceed anyway
Hope this helps?