Convert PFX to PEM and upload the certificate to Plesk

Export the Private Key:

  openssl pkcs12 -in filename.pfx -nocerts -out key.pem

Remove the password from the SSL certificate (unencrypted is needed for plesk):

  openssl rsa -in key.pem -out server.key

Export the certificate:

  openssl pkcs12 -in filename.pfx -clcerts -nokeys -out cert.pem
  
Extract the CA Root:

  openssl pkcs12 -in certificate.pfx -out certificate.cer -nodes