Can my client download the form data (csv) without have to become user?

There are many different ways in which you can do the setup depending on your client's needs.
We recommend that you create a 'Workspace' and invite your client. With this clients can create new forms, view submissions reports, visitors reports, etc. So they get all features inside FormNX to use inside the workspace. Inside this workspace the client can add more collaborators or team members also. So if you've 20 workspace you can invite 20 different clients.
Alternative ways is to sync data with Google Sheet or use webhook to capture data in some other system for using, which is not a direct way.