I got a new Heron installation and had to reinstall everything. Hope it works for you. Just open up a terminal and enter these commands.
echo "deb http://packages.medibuntu.org/ hardy free non-free" | sudo tee -a /etc/apt/sources.list
wget -q http://packages.medibuntu.org/medibuntu-key.gpg -O- | sudo apt-key add - && sudo apt-get update
sudo apt-get remove mozplugger && sudo apt-get install acroread acroread-plugins mozilla-acroread mozplugger flashplugin-nonfree
Thanks! Got confused after looking elsewhere, but this worked a treat for me.