I always have to look this up... hence the post here...
Even though you might have a key file in text form, it won't work with AWS if you need to connect from ssh command line (keys work fine from Putty / WinSCP).
To convert a key file you also need the crt file.
Here's how from Mac / Linux command line:
openssl rsa -in server.key -text > private.pem
openssl x509 -inform PEM -in server.crt > public.pem
Once you have the pem, don't forget to:
chmod 600 public.pem
Otherwise you won't be able to use it to ssh to AWS.