Execute the following 3 commands in a terminal window on your machine :

sudo wget http://www.medibuntu.org/sources.list.d/gutsy.list -O /etc/apt/sources.list.d/medibuntu.list

wget -q http://packages.medibuntu.org/medibuntu-key.gpg -O- | sudo apt-key add - && sudo apt-get update

sudo apt-get install acroread

Or you can install through Applications - Ubuntu Software Center, if your system is 10 or higher.