If you are talking about a Visa or Mastercard debit card, to PayPal, that's seen and treated completely like a credit card.

Paypal will always default to your bank account. You have to change this for every transaction. You need to manually choose to use your Credit Card (which we know is a debit card ), and then you still have the choice of whether PayPal or your debit card does the currency conversion. You have to look at your debit card documentation to see their currency surcharge (typically the same 2.5% as PayPal nowadays).

However, just like a credit card, if you choose to have your debit card due the conversion, you won't be assessed the rate at the day of the transaction. Rather, the 1-3 business days later when the money is actually taken out of your account. It's up to you to know when/how to use this to your advantage.