Converting Putty ppk file to pem in Ubuntu

First you need to install putty tools. You may either install putty from the Software Center or use the following command in terminal

sudo apt-get install putty-tools

Converting the ppk file to pem file 

Use puttygen to convert ppk to pem

sudo puttygen /path/to/ppk/file.ppk -O private-openssh -o /path/for/saving/pem/key.pem

Copy the key.pem to ~/.ssh path 

sudo cp /path/of/pem/key.pem ~/.ssh/

Set file permission for ~/.ssh/key.pem 

sudo chmod 400 ~/.ssh/key.pem 

Use the pem key to ssh to the remote system 

sudo ssh -i ~/.ssh/key.pem