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Hello,
It doesn't appear on CMC which means that it is either new or has a low trading volume, which doesn't make it worth taking note of. Why do you want to use this particular exchange and not one of the reputable ones?

It has a great offer exchanging BTC for PAX, but I don't know if I can trust them. Thanks for your answer.

Take a look here. https://coinmarketcap.com/currencies/paxos-standard/markets/
Most PAX trading volume comes from BitZ. You can also trade it on Binance. I would suggest you don't send any money to an exchange you don't feel comfortable with.

Whois records show that the site started its operations in November 2019. They are brand new in this game.