Basically when you working on windows, you most likely use putty to connect to the server. Putty generates the ppk file as the private key.
But the ppk file cannot be using to connect classic linux server or ec2 in AWS. You need to convert this ppk file to pem first in order to use it.
$ sudo apt install putty-tools
for centos and other linux:
$ sudo yum install putty
Start converting the ppk to pem
$ puttygen key.ppk -O private-openssh -o key.pem
And you might want to set the permission
$ chmod 600 key.pem
Now the key.pem is ready to use.