crops 50 pts from the left, top, right, bottom (in this order).
If you run only the command pdfcrop input, it will output a file titled input-crop.pdf with zero margins. I find this very handy when including pdf illustrations in documents.
Cropping multiple files
Unfortunately, pdfcrop cannot crop multiple files at the time. It is however easy to write a script that will crop all pdfs in the folder the script is located in.
Create a new empty file, and call it something.sh. Open it with a text editor and insert the following:
for FILE in ./*.pdf; do
Save it, and close. Then right click the file, go to Properties > Permissions and check the field Allow executing file as program. Now close the dialog. Run the script by double clicking it and choosingRun in Terminal. And new, zero-margin cropped version of all pdfs with suffix -crop will now be printed in the folder. If you want margins or other things, you can of course just open the script and add arguments after pdfcrop.