Unfortunately, when providing digital goods you are susceptible chargebacks & disputes. To combat this, we have Download Tracking in Dashboard My Sales (click the blue order number)
This shows the date, time and ip address of the download and should be shown to Paypal/Stripe when you have a dispute. The process varies slightly, depending on how they are disputing the charge, but generally you will have to take a screenshot showing the download, then respond to the dispute in Paypal/Stripe and upload the screenshots, and explain that the file has been downloaded
This information is also logged on the contract after purchase, and an electronic signature (E-Signature) is generated for both the customer and seller, signaling the acceptance of terms and acknowledgement of the purchase. This should also be shown as evidence to Paypal.
Chargeback vs Dispute
Chargebacks are a lot harder to win, as the decision is made by the buyer's bank/card issuer, and it's very rare that they will decide against their own customer, even if you've provided a tonne of proof. So try not to get too down if you lose those, they are nearly impossible to win. (They also come with an extra charge - around $25, so you can expect to see that as well)
Disputes are a lot fairer to the seller, as the decision is made by Paypal. Paypal have no reason to be biased, and will decide based on the evidence provided and are a lot easier to win. And if you do lose, you do not have an extra $25 charge on top