Same as others, everything. I have an smtp rule that sends a copy of any PDF I receive to that other mailbox and adds it to paperless. I also bought a little Epson scanner to do mass scanning of all the paper documents I had.
It's just super useful to tag invoices and whatnot as "taxes 2023" and when it's time to send it to my fiduciary, I just have to search for that tag, download it all, zip it and send it over.
Just takes a little bit of discipline and a bit of time here and there to scan documents I received by regular mail and add them to paperless.
Probably one of the most underrated selfhosted app if you ask me in terms of making life easier.