Install PuTTY tools:

sudo apt install putty-tools

Convert .ppk to .pem using putty-tools:

puttygen jenkins-server.ppk -O private-openssh -o jenkins-server.pem

Set correct permission for key file:

chmod 400 jenkins-server.pem

Connect to server using .pem key file via ssh:

ssh -i server1.pem [email protected]