I luckily have a professional document scanner for this, which has wifi and is able to store the PDF on a cloud drive. Started using ASN numbers recently, so I put a sticker on the letter before scanning. The letter gets archived and is found quickly if needed because of the ASN.

Yesterday I wanted to introduce syncthing as my main backup solution. However, I am struggling with setting up the discovery server and want to fix that before installing it on all my devices.