I'm using Genius Scan+ on my phone and bought the cloud backup option for like $3 one-off, that enables automated exports to dropbox, google drive and a bunch of other services. Every document I receive is scanned and adequately named right away, and then automatically exported to both google drive and dropbox.

The dropbox client then again runs on my laptop and desktop and automatically syncs new files to the local folders, so I have the original scan on my phone plus two cloud backups and the local copies of the cloud backups on another two devices.

The original documents are kept in physical folders, neatly stored at home.

In case the important document is a digital copy only, I will export it from my mailbox directly to the dropbox & google drive, so it's the same as above minus the copy on my phone. Depending on how important it is, I might also print a copy for safekeeping and/or forward it to a secondary email should I ever lose access to my primary.