In PKI, the main certificate file formats are:

1. PEM (Privacy-Enhanced Mail): Widely used for CA certificates, it stores certificates and keys in Base64-encoded format with clear headers (-----BEGIN CERTIFICATE-----). Common in Unix-based systems.
2. DER (Distinguished Encoding Rules): A binary format without headers, often used in Java environments and certain hardware appliances.
3. PFX/P12 (Personal Information Exchange): Combines certificate and private key in a password-protected binary format, widely used in Windows environments.
4. CER/CRT: This can be in PEM or DER format and typically contains only the certificate.
5. PKCS #7, or Cryptographic Message Syntax (CMS), is a standard for encrypting and signing data. It allows the secure exchange of information in various formats, enabling functionalities like digital signatures, message encryption, and certificate handling. PKCS #7 files typically use the .p7b or .p7s file extensions can encapsulate multiple certificates and signatures, making it suitable for secure communications in PKI environments. For more in-depth information, you can check relevant sources on the topic.

Each format supports different PKI use cases depending on system compatibility and security needs.

Here are the standard file extensions for the main certificate formats used in PKI:

• PEM: .pem, .crt, .cer
• DER: .der
• PFX/P12: .pfx, .p12
• CER/CRT: .cer, .crt (may also use PEM or DER)

These extensions indicate the encoding or format used for the certificate files.