It seems my
/etc/apt/sources.list file only has
deb cdrom:[Linux Mint 17 Qiana - Release amd64 20140624]/ trusty contrib main non-free
It might have got corrupted when I was trying to download
pdfedit. When I try to download some software now it says:
This is a major failure of your software management system. Please check for broken packages with synaptic, check the file permissions and correctness of the file '/etc/apt/sources.list' and reload the software information with: 'sudo apt-get update' and 'sudo apt-get install -f'.