I'd like to be able to send someone a file e.g. a document.
I don’t think this is supported and I know it’s always been contentious to do so because as a forum admin you would notionally be liable for people that use it to send illegal files.
I don’t know how Facebook or Discord deal with that on a legality front though.
Wealth and Power!
No you can't send files via the PM system. A typical admin would not know that their site was potentially being abused.