Obtaining API Credentials
To use the PayPal API, you must have API credentials that identify you as a PayPal Business or Premier account holder who is authorized to perform various API operations. Although you can use either an API signature or a certificate for credentials, PayPal recommends you use a signature.
Important: Even though you can have both a signature and a certificate, you cannot use both at the same time.
To learn more about creating Classic API credentials, see Creating and Managing Classic API Credentials.