As for our US users: yes, you can. You can withdraw your earnings, but you can also buy documents with it. If you want to buy a summary, choose the 'Stuvia credit' option in the shopping cart. The advantage of paying with your Stuvia credit is that you do not have to pay for any transaction fees.
As for our UK users: unfortunately this is no longer possible. New European regulations, the Payment Service Directive 2 (PSD2), require Stuvia to take a different approach when it comes to payments. One of the consequences is that it is no longer possible to pay for your purchases with earned credit.