How to make a workflow with notifications for a budget organization?

Question

Document flow

The task is this: there are several budgetary organizations and there are a couple of dozen condominiums.
Monthly it is necessary to bring reports to the organization (now I'm just going).
It is necessary to automate this activity, to make electronic document management with them.
Because organizations are budgetary, so their computers are correspondingly depressing.
It is also necessary to have the possibility of electronic signatures.
How I see it for myself so far: cloud hosting (dropbox? owncloud/nextcloud? webdav?) upload PDF with gnupg/gpg/pgp signature?
Or even send by e-mail attachments with S/MIME? ZeBat already exists on the other side (judging by the headers of the letters they send).
So, it is necessary that it be:
free or its own cloud
, which on ancient pentiums can see incoming files
- it was convenient for us to post reports for them
- to sign
- and for them, as for "geniuses", there were some notifications that reports had arrived today
- it would be desirable to be able to put some notes on them (OK / garbage remake "), so that you could control that the reports were submitted , or "something is wrong, everything is lost, we will all die."
What do you advise?