Create a new format ".PDFPK" that is in every other respect a normal PDF document, but encrypted with a public key. Also have Libreoffice generate and manage public key / private key pairs.
Mortgage company generates a key pair and releases public key on their web site. Users can then generate a document and encrypt it with their public key as a .PDFPK . Then it can be sent via regular EMAIL securely, and the bank is the only entity that can turn it into a regular .PDF .
I don't know if this will be a solution that's quantum hard,
but this may be a chance to make Libreoffice the leading edge, making M$ play catch up.
You can use PGP (or any another) encryption for your PDF files now. Also you can set up a password for open/edit your PDF file
(In reply to Roman Kuznetsov from comment #1)
> You can use PGP (or any another) encryption for your PDF files now. Also you
> can set up a password for open/edit your PDF file
Closing as RESOLVED WONTFIX