Instructions for Ubuntu 14.04LTS (and more than likely others). Where keyfile.ppk
is the PPK file you have in hand.
$ sudo apt-get install putty-tools
# create public key as [id_rsa.pub]
$ puttygen keyfile.ppk -o id_rsa.pub -O public-openssh
# create private key as [id_rsa]
$ puttygen keyfile.ppk -o id_rsa -O private-openssh