.ppk files are created by PuTTY to store a private key generated by the program.
To convert keypairs, first you need to install the package putty and then:
Generate the private key:
puttygen id_rsa.ppk -O private-openssh -o id_rsa
Generate the public key:
puttygen id_rsa.ppk -O public-openssh -o id_rsa.pub