As of July 2017, PayPal does not accept MasterCard directly as a payment method. MasterCard users can link their credit or debit card to their PayPal account and use it that way, but they cannot simply enter their MasterCard information when prompted by PayPal. This is true for both the MasterCard credit card and the MasterCard debit card.
I do not have personal experience with this, but from what I can find online it appears that you can use a MasterCard with PayPal.
PayPal does accept Mastercard services as of right now. This means that you can use your Mastercard to make purchases through PayPal. This is a great way to use your Mastercard, as it allows you to make purchases without having to carry around cash or a checkbook.