There are two major scenarios with PayPal transactions, which causes some confusion:
You pay without being logged into a PayPal account
In this case PayPal are acting as a normal card payment processor, and chargeback rights apply as normal (Section 75 also, for credit card purchases)
You pay while logged into your PayPal account
In this case between your account and PayPal you normally have an “account funding transaction” (topping up your PayPal balance). Normal chargeback rights do not apply - they only go as far as the point the funds were credited to your PayPal balance (before being immediately debited again)
Now, in practice PayPal do not contest a lot of chargebacks they could theoretically win even in the second case (I think the reason for this is that their systems seem to be inconsistent about properly flagging them as account funding). However, they will almost always terminate your account and ban you from opening another if you raise a chargeback against them for a transaction made while logged into a PayPal account.
Similar considerations apply if you pay PayPal via Direct Debit, for example (also, if you do so, and this is for a payment to a non-GBP merchant, expect them to sting you on the exchange rate)