1. Copy the PDFs you want some folder
2. Open Terminal in the folder or make a script and run the script in the folder.
.. the script
#! /bin/bash
convert -density 200 -trim *.pdf -quality 100 picture.jpg && mkdir PICTURES && find -iname ”*.jpg” -exec mv {} PICTURES \; && convert -density 85 -trim PICTURES/*.jpg -quality 75 picture.pdf && rm -rf PICTURES
.. make script executable
chmod +x convert.sh
Edit: If conversion does not work in Mint or an another Ubuntu-based distro, the reason is faulty /etc/ImageMagick-6/policy.xml
file. In PClinuxOS has a workable file.