This is useful if you have a bunch of images/scans where the same area at the top needs to be chopped.
Sometimes the -trim option (such as the case of white space on top) is wrong and ends up cropping the image and losing some text.
ImageMagick's convert is very useful but tricky to find some things out as I find it to be less than intuitive sometimes.
Chopping off the bottom
convert -crop -0-400 image.jpg image.pdf
The -crop -0-400 is what does it. The 0 says not to chop anything from the left. If you want to chop the left of course change 0 to whatever pixels you want chopped from the left. The 400 says to chop 400 pixels off the bottom.
Chopping off the top
convert -crop +0+200 someimage.jpg output.jpg
This chops off the top "200 pixels". Adjust accordingly.