To view .cer files

openssl x509 -in cerfile.cer -noout -text
unable to load certificate
1232435:error:09FFF06C:PEM routines:CRYPTO_internal:no start 
line:/System/Volumes/Data/SWE/macOS/BuildRoots/b8ff8433dc/Library/Caches/com.apple.xbs/Sources/libressl/libressl- 75/libressl-2.8/crypto/pem/pem_lib.c:684:Expecting: TRUSTED CERTIFICATE

Solution, try:

openssl x509 -inform der -in cerfile.cer -noout -text
openssl x509 -inform pem -in cerfile.cer -noout -text

See also

• Certificate, CSR (PKCS10), /etc/letsencrypt/csr/, openssl req, X.509, [.pem, .cer, .csr], Kubernetes CertificateSigningRequest