Do I need a certificate

All Dropbox API calls must be made over SSL/TLS, so your app needs to be able to make SSL/TLS connections, but it doesn't need its own certificate. The Dropbox API servers themselves are served with their respective certificates, which your client just needs to trust.

So, client does not need a certificate to download a file ? 

Thanks in advance!

asierdo That's correct, the client does not need its own certificate to download files from the Dropbox API. The Dropbox API servers have certificates, and third party clients connect to those Dropbox servers.