I'm Miles from the Social Care team at Shopify.
I'm going to give you a broad answer to your question; but for PayPal's specific process, you'll be best to reach out to them directly.
Chargebacks occur when a customer contacts their bank or credit card provider to reverse a transaction on the account. A customer can do this without contacting the business first, which can cause stress for a merchant. We have a simple setup for managing chargebacks, which I've outlined below.
If a chargeback is raised on a Shopify order, you'll receive a notification in your order dashboard. Inside this notification, you will be able to submit evidence that you either ship the product; have a policy stating why you haven't shipped a certain product or even communications with your customer if those communications exist. Once a chargeback is raised, we'll hold the disputed amount until the chargeback is completed.
From there, the customers' bank will investigate the customers' claim and your evidence before making a decision. If the chargeback is successful, the customer will receive their money back. If it is unsuccessful, we'll release the funds back to you.
To answer your initial question; if a customer raises a chargeback, Shopify (and other providers) are required to raise the investigation. This means we could not reject a chargeback, but you'll have ample time to submit evidence for the chargeback.
As I mentioned above, for PayPal specific information, I would suggest reaching out to them. In the meantime, I have found PayPal's chargeback FAQ's which might help you answer that question: https://www.paypal.com/my/webapps/mpp/security/sell-chargebackfaq.
If you have any questions at all, please let me know.
Have a great day!