How to convert your certificate to the PEM format?
The web server software (Apache) only supports PEM certificates. To convert certificates to the PEM format, an online convertor can be used, or on Linux systems OpenSSL can be used to convert certificates in a different format to PEM:
Convert x509 to PEM
openssl x509 -in certificate.cer -outform PEM -out certificate.pem
Convert DER to PEM
openssl x509 -inform der -in certificate.cer -out certificate.pem
Convert PFX to PEM
openssl pkcs12 -in certificate.pfx -out certificate.pem
Convert PKCS7/P7B to PEM
openssl pkcs7 -print_certs -in certificate.p7b -out certificate.pem