There are a lot of ways you can exchange bitcoin to paypal balance, the first and most secure way is to use and exchange site like poloniex, where you can trnasfer funds from bitcoin to paypal  and vis versa, another way would be to find somone who is willing to buy bitcoin in exchange for paypal balance, and you can find many offers on peer to peer exchange sites like localbitcoin, and in my opinion it would be the most intracebale one.