puttygen key.pem -o key.ppk
1.install putty-tools with the following command:
sudo apt-get install putty-tools
2.Now convert your .ppk file to .pem using the following command:
puttygen yourkey.ppk -O private-openssh -o yourkey.pem
3.Set the proper permission to use the .pem file with following command:
chmod 400 yourkey.pem
4.Connect to you server using the following command
ssh -i yourkey.pem serverusername@server-ip