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........