How to create a cer file from a pfx

If you want to extract client certificates, you can use OpenSSL's PKCS12 tool.

openssl pkcs12 -in input.pfx -out mycerts.crt -nokeys -clcerts

The command above will output certificate(s) in PEM format. The ".crt" file extension is handled by both macOS and Window.
You mention ".cer" extension in the question which is conventionally used for the DER encoded files. A binary encoding. Try the ".crt" file first and if it's not accepted, easy to convert from PEM to DER:


openssl x509 -inform pem -in mycerts.crt -outform der -out mycerts.cer