I am a small accounting firm. One of my clients upgraded to 2018 Enterprise version, Can I copy the files to 2018 Pro version
no, only enterprise or enterprise accountant can open enterprise files
pro and premier can open each others files
You can convert the Enterprise files to Pro/Premier using our conversion service. See BRC Enterprise to Pro/Premier Conversion Service
This makes sense if you'll be taking over the books for the client (for example) or will rarely need to convert the file. Or if the client doesn't really need Enterprise and also wants to convert to Pro/Premier.