If a pdf file is very old, using acroread, convert(print) it to a ps file that can be opened with the emacs. For example, it is called "output.ps"
The letter size is Pts (H 792 W 612) and I want to crop left 60, right 62, top 54, bottom 70 pts. So the CropBox option is
* Note that unfortunately, the Amazon kindle opens this modified pdf extremely slow.