Convert p7b Certificate File to pem

Converting Your Existing Certificate To PEM Format

If your certificate is not in PEM format, you can convert it to the PEM format using the following OpenSSL commands:

Convert DER to PEM

openssl x509 -inform der -in certificate.cer -out certificate.pem

Convert P7B to PEM

openssl pkcs7 -print_certs -in certificate.p7b -out certificate.pem

Convert PFX to PEM

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

Source: https://support.aerofs.com/hc/en-us/articles/205007260-How-Do-I-Convert-My-SSL-Certificate-File-To-PEM-Format-