Ghostscript is a great tool for reducing pdf file size.
On a Debian-based system, make sure to have the ghostscript package installed.
To make the file reduction a reality, run the following command line :
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -dNOPAUSE -dQUIET -dBATCH -sOutputFile=new_file.pdf current_file.pdf
There are several options for -dPDFSETTINGS :
/screen : low-resolution, lowest output size
/ebook : medium-resolution, medium output size
/printer OR /prepress : high-resolution with maximum output size