There are several file sharing protocols, and several implementations. One way of many to solve your problem is to install an ftp/sftp server on the PC that needs to share the files (e.g. [FileZilla][1]FileZilla).
You can then use port forwarding rules on your router to make that server accessible from the internet, if you want/need to. By default, the router will prevent access to the ftp server from the internet, unless you manually change it.