First of all, add Medibuntu repository in your sources list. This can be done via:
sudo wget http://www.medibuntu.org/sources.list.d/hardy.list -O /etc/apt/sources.list.d/medibuntu.list
Once this is done, add Medibuntu GPG key.
sudo apt-get update && sudo apt-get install medibuntu-keyring
Then update the list of available packages by
sudo apt-get update
Finally, install Acrobat Reader using
sudo apt-get install acroread