If you don't pay the outstanding balance you won't be able to use PayPal any longer. I doubt that they would take legal action against you for 25 euros, but it will go down on your credit reports and make life much more difficult for you in the future. Do you think a bank, or any other financial institution, would give a loan to someone with unpaid debts on their credit record?
Be pragmatic and clear the debt; it's not worth the hassle for just 25 euros. If you have concrete evidence that you supplied the article to the person in question (difficult to prove with a digital item) you could fight the case with PayPal but you're unlikely to win.