If you want to get a Proton Email users public key so you can encrypt your messages with them, and don't use Proton mail, here is how you can download their public key from Protons key servers.

Method 1: If you want a simple method, I made a tool at Source code can be viewed with a button on the website.

https://protonkeyfetcher.privacyjam.com/

You enter an email, and it gets it for you with the option to copy and download it.

Method 2: Go to https://api.protonmail.ch/pks/lookup?op=get&search=emailhere where emailhere is the email, so something like

https://api.protonmail.ch/pks/lookup?op=get&search=contact@proton.me.

This will give you the .asc file.