.PPK File Extension
What is a PPK file?
File created by PuTTYgen (PuTTY Key Generator), a program used for generating keys for public-key cryptography; stores a private key generated by the program; used for enabling secure communication with a remote entity that has the corresponding public key.
PPK files are used by PuTTY, a free SSH and Telnet client. To set the PPK file for authentication, choose Connection → SSH → Auth from the PuTTY Configuration menu and then select Browse... for the "Private key file for authentication" option.
NOTE: PPK files can also be opened using WinSCP, an SFTP and FTP client for Windows. You can set the PPK file location when creating a new connection in WinSCP by selecting Advanced → SSH → Authentication → Private key file.