A PKI certificate is needed when you want to use a digital signature. You may hear PKI being referred to as public key infrastructure, when it is not abbreviated. This type of certificate can contain a public key or a private key. The type of key required depends on whether you are creating electronically signed documents, or receiving them for authentication.
An Electronic Signature Does Not Require a PKI Certificate
It should be pointed out that an electronic signature does not require a PKI certificate. This is because an electronic signature is not encrypted and, therefore, does not require a public key or private key. People often get the terms electronic signature and digital signature confused, or assume they are the same thing. When you need to encrypt your signature in electronic documents, you need to use a digital certificate, which is part of public key infrastructure certificates.