How do I convert a .pem file into a .ppk, and vice versa

Step 1: Check for "puttygen" version

Step 2: update the "apt" package

Step 3: Now install the "puttygen" tools by typing the below command

Step 4: Again check the "puttygen" tools version 

Steps to Convert the ".ppk" to ".pem"

sudo puttygen ppkkey.ppk -O private-openssh -o pemkey.pem

-O (Capital Letter O) private-openssh
-o (Small Letter o) pemkey.pem

Open the terminal from "ubuntu-aws.ppk" file container folder

Now, verify the connection with aws-ec2 instance using converted "ubuntu-aws.pem" by typing the below command

ssh -i </path/of/your/pemfile.pem> <username@host> and type [yes] to establish the connection

Eg: ssh -i /.ssh/ubuntu-aws.pem ubuntu@ec2-18-221-121-111.east2-amazonaws.com and press [Enter]

and type [Yes] to establish the connection

Unix or Linux - convert a .pem file to a .ppk file

On the instance shell, run the puttygen command to convert your .pem file to a .ppk file:

sudo puttygen pemKey.pem -o ppkKey.ppk -O private

Link : https://aws.amazon.com/premiumsupport/knowledge-center/convert-pem-file-into-ppk/