In the case of using 'filesDownloadZip', that data is the actual raw data for the zip file containing the requested folder.
It looks like you have the Dropbox part of this itself working, so I don't think I can offer to much help with the part that you're currently stuck on. Exactly what you do with that returned data and how you serve it back to the user is up to and will depend on the web framework you're using in this case.
That said, I wouldn't expect to use '.json()' or 'JSON.stringify' on the returned data, as it's not an object you necessarilly want to convert into a JSON string. Also, 'window.open' doesn't seem right as that would expect a URL, but the data isn't a URL.