.KEY + .CRT => .PEM

.key (optional):
<number>.crt *OR* www_domain_com.crt:
SectigoRSADomainValidationSecureServerCA.crt:
USERTrustRSAAAACA.crt:
 
.KEY + .CRT + .CA_BUNDLE => .PEM

.key (optional):
.crt:
.ca_bundle:
 


.PFX => .PEM using OpenSSL: (Download OpenSSL binaries)
openssl pkcs12 -in certificate.pfx -out certificate.pem


.P12 => .PEM using OpenSSL: (Download OpenSSL binaries)
openssl pkcs12 -in certificate.p12 -out certificate.pem