Hello,
A few days ago, the GPG public key used to check the Flatpak repository
signature has expired. When updating Paperwork, you will get the error
message "Error: GPG signatures found, but none are in trusted keyring".
I had to generate a new public key. In order to get this new key, I
think the simplest for both me and you is to reinstall Paperwork:
flatpak --user remove work.openpaper.Paperwork
flatpak --user install
https://builder.openpaper.work/paperwork_master.flatpakref
Don't worry about your documents: When uninstalling Paperwork, your
documents are never deleted.
Sorry for the inconvenience,