Download Page Speed from here
Lets get started.
dpkg -i mod-pagespeed-*.deb
apt-get -f install
Now we need to configure pagespeed.conf file which is located in /etc/apache2/mods-available/ to turn it on.
On the top of pagespeed.conf file you will see the following:
Do not do anything if the line ModPagespeed is on like in above example. If it’s off than change off to on, save file and restart apache.
For more configuration options I recommend you to read more here https://code.google.com/speed/page-speed/docs/using_mod.html