Well, technically you could store the private key or seed phrase for a Bitcoin wallet in a PDF file. But it’s not something I'd recommend.

You see, keeping your keys in a digital file like that makes them vulnerable to theft. If someone gets their hands on that PDF, they could access and drain your wallet.

A safer bet is to write down your keys or seed phrase on paper and keep it somewhere safe - old school style!

Always remember the golden rule of Bitcoin: Your keys, your coins; Not your keys, not your coins!