What do you mean buy and exchange? Are you referring to buying a product with the paypal balance then exchanging (returning) it for cash?
If you add a debit card to your paypal account, then you can shop using your account, buy and exchange any currency, you can even transfer the currency you have bought to your bank. In other words, you can do every if you atleast add credit or debit card to paypal.
The context of "buying" here is buying a currency. So, for example, you buy euro or singapore dollar or baht or anything. "Exchange" here refers to exchanging the currency to other currency.