If you are not able to login to a Linux server from putty using a ppk file, you can regenerate the file using the below options.
Open puttygen and Make sure RSA is selected under the Parameters section.
Click the load button (Load an existing private key file)
Select the .pem file in the load private key dialog box. make sure All Files *.* is selected to see the .pem file.
you will see a dialog box, click ok
from the key menu at top, select Parameters for saving key file, select the "PPK file version 2" option and click ok
Click the Save Private key and click Yes in the puttygen warning dialog box. save the key and authenticate using it.
