Ghostscript

ghostscript⚑

Compress PDF document⚑

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -dNOPAUSE -dQUIET -dBATCH -sOutputFile=output.pdf input.pdf

"/screen" is a very poor quality. For scanned documents "/ebook" works great.

(https://askubuntu.com/questions/113544/how-can-i-reduce-the-file-size-of-a-scanned-pdf-file)