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.