Which credentials to use for API

I am a developer and have a client who wants to automate password protected PDF creation. I already have the PDFs being generated automatically through our Google environment, but want to find out what's involved with password protecting them through the API.

I see I can get developer credentials, but once it's delivered, how do I set it up so that the person using the tool (my client) will be passing along their credentials.

I also see dev credentials expire after 1 year. Is the same true of user credentials? If so, I'd need to inform my client about renewing them.

Can someone direct me to an article or briefly answer here?

Thank you,

Steve