We automatically upgrade your account to Premium if someone has signed up for Kopernio using your unique referral link e.g. https://kopernio.com/invite/yourreferralcode . You can find your referral link when you view a PDF in the Kopernio viewer.
The premium account comes with 2 GB of storage, unlimited tagging and the option to mirror your Locker to Dropbox. Please visit https://kopernio.com/premium for more details.