Puttygen should also be able to load the .pem file using the Conversions - Import Key menu item.
"Textastic expects the key file to be in the format used by the OpenSSH utility "ssh-keygen". If you instead have a key in PuTTY's format (.ppk), you can use PuTTYgen to convert it to OpenSSH format:
Visit the PuTTY website and download puttygen.exe (on your PC). Open it, and using the File menu, load your .ppk that you already use, and type in your passphrase. Now go to the Conversions menu and export the key as an OpenSSH key. You can now use the converted key with Textastic."
You can use a command line like this to import (-i) your pem file and convert to OpenSSH format:
ssh-keygen -i -f infile.pem > outfile
Customer support service by UserEcho