Refer: https://www.baeldung.com/linux/pem-ppk-keys
sudo apt-get install -y putty-tools
Private
puttygen id_rsa.pem -O private -o id_rsa_private.ppk
Public
puttygen id_rsa.pem -O public -o id_rsa_public.ppk

Technical notes, and other ideas.
Refer: https://www.baeldung.com/linux/pem-ppk-keys
sudo apt-get install -y putty-tools
Private
puttygen id_rsa.pem -O private -o id_rsa_private.ppk
Public
puttygen id_rsa.pem -O public -o id_rsa_public.ppk