If new Client was using Enterprise accountant version 19 and I am using Premier Accountant 2018 version 28, can I use their file or does it have to be saved differently?

No, you cannot use their file.  Enterprise and Premier use totally different databases.  Also, you could not open a 2019 file with a 2018 product.  You would need to either purchase Enterprise 2019, or have him purchase Pro/Premier/Accountant (any 1 of the 3) 2018.

There are services that will convert an Enterprise file to one of the other 3 formats, but none that I know of that can convert it down a year.

For details, check with BigRedConsulting.com.