Because PAX is an ERC20 token, every transaction with it takes place on the Ethereum blockchain and therefore requires that the sender pay a fee in ETH to get the transaction confirmed. Learn more about this fee here.
You will need a small amount of ETH in order to Swap your PAX for any other cryptocurrency within the Blockchain.com Wallet or to send it externally to a third-party exchange or platform. However, you do not need to keep an ETH balance in order to receive or to store PAX in your Wallet. If you don’t have enough ETH to use your PAX, you can Swap for it from another cryptocurrency or receive it from another exchange or platform.