Normally, the certificate you created from the iOS developer center is a .cer
format certificate.
After you install this .cer
format certificate, you can export the certificate with a format of .p12
from keychain.
At last you can use the following command to convert the .p12
certificate to .pem
certificate for some tools such as Houston.
openssl pkcs12 -in cert.p12 -out apple_push_notification.pem -nodes -clcerts
Reference:
Houston