Can I delete files on Mac that are uploaded to Dropbox and not lose them?

First, to clear up any confusion, the Dropbox folder on your computer is a folder like any other. Anything you put in the folder takes up space on your drive. That means that if you copy a file into the Dropbox folder, it now exists in two locations (the original folder and the Dropbox folder) and is taking up twice the space. For this reason, anything you want to store in Dropbox should be moved to the Dropbox folder. Also, Dropbox will only sync the files that you place in the Dropbox folder. It cannot sync any other folders on your computer automatically.

Now that that's out of the way... if you just want to store files online without them taking space, you can use Selective Sync or Smart Sync.

Selective Sync, available to any account level, allows you to selectively remove folders from your local drive while keeping them in your account online. Once a folder has synced (and NOT before) you can uncheck it in Selective Sync and it will be removed from your local drive. You would then use the Dropbox website to interact with that folder or download it as needed.

Smart Sync, available on Professional and Business accounts, allows you to set files and folders as Online Only. They still appear on your computer but they don't take up space. If you try to open a file that's marked as Online Only, it's first downloaded to the computer and then opened automatically.