Thought I would just weigh in on this, just gather as much evidence as you can in your favour. If you have a policy for refunds as I see you are talking about an E-Book, you can state that once the person has paid and received the PDF then they have immediate access to the content and refunds are not possible.
I manage an SMM panel, with close to over $1 Million USD in sales all through PayPal over the course of 3-4 years. I have won EVERY single case against refund disputes, as I will write out a response to the dispute outlining everything: All conversations with the client, orders made by the client, refund policy of the website and in cases where they say it wasn't them who made the payment, evidence of the email they paid with and how it is linked to them. Even in cases proving that they have made orders to social profiles linked to them.
I see a lot of people here saying that they will side with the buyer, but in most cases you can provide a lot more evidence to the contrary of the refund request, its how well you can write the dispute response and also remaining polite and professional in doing so.
I saw also someone mentioned to phone PayPal, I can second that is also a great way to get the dispute resolved in your favour if you can explain your policy regarding the sale of your E-book and the immediate access to it once purchased. (If you don't have this as a policy already I would suggest doing so)
After a while of having a PayPal account they will not take the money from your account on hold when a dispute is made, but im unsure after how long this takes and think it entirely depends on your transaction volume and case history.
Best of luck to with your case and hope that you emerge victorious!