PEM (originally “Privacy Enhanced Mail”) is the most common
format for X.509 certificates, CSRs, and cryptographic keys. A PEM
file is a text file containing one or more items in Base64 ASCII encoding,
each with plain-text headers and footers (e.g. -----BEGIN CERTIFICATE-----
and -----END CERTIFICATE-----). A single PEM file could contain
an end-entity certificate, a private key, or multiple certificates
forming a complete chain of trust. Most certificate files downloaded
from SSL.com will be in PEM format