If you want to reduce PDF file size using command line you can use ghostscript.Ghostscript is used for PostScript/PDF preview and printing. Usually as a back-end to a program such as ghostview, it can display PostScript and PDF documents in an X11 environment.
Furthermore, it can render PostScript and PDF files as graphics to be printed on non-PostScript printers. Supported printers include common dot-matrix, inkjet and laser models.
Package gsfonts contains a set of standard fonts for Ghostscript.
Use the following command syntax to reduce your pdf files
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -dNOPAUSE -dQUIET -dBATCH -sOutputFile=output.pdf input.pdf
Replace your file names output.pdf,input.pdf files with your file names.
This is only example you can check
man gs command for more options.
If you want to install ghostscript use the following command
sudo apt-get install ghostscript