Most Linux distributions have PuTTY (package name
putty) available for Linux. You could install PuTTY on the Linux side and use
puttygen to convert the .ppk files to the regular
ssh style key files (called PEM files - even though they don't get a .pem in the file name).
puttygen id_dsa.ppk -O private-openssh -o id_dsa
NOTE: You can also use
puttygen to import
ssh style PEM files back into PuTTY.
PuTTY's author opted for simplicity, so the public and private keys, which make up the underlying security used by PuTTY/SSH-2 key authentication, are stored in a single proprietary .ppk file. Typically these keys are maintained as two separate files by
On Linux the key files are typically kept in the directory .ssh.
There is a good overview of the conversion process here in this Stack Overflow question titled: Convert PEM to PPK file format.
The author of PuTTY also discusses his rationale for using .ppk files in the PuTTY users manual. You can read about it here in section 8.2.12.