public key infrastructure – SFTP File to Server Without Local User Access

Need to allow outside OpenSSH client to SFTP file to Reflection Secure IT Server without the need for local user’s PKI.
In the past, I had a server with a local user account.
The client gave me his public key and I authorized it, but this requires a local user account on the server.
I am trying to find a method to allow the same without the need for a local user account.
I assume public/private keys only work when there is a local user account to connect as.
Is there a way to use a CA Trusted Certificate for this?
If so, any help on how that works would be appreciated.