<We have an application subjected do sign documents and store them somewhere.>
I developed a relative simple "file archive" with PostgreSQL (web application with JSF for user interface). The major structure is one table with some "key word fields", and 3 blob-fields (because exactly 3 files belong to one record). I have do deal with millions of files (95% about 2-5KB, 5% are greater than 1MB). The great advantage is that I don't have to "communicate" with the file system (try to open a directory with 300T files on a windows system... it's horrible, even on the command line).
The database now is 12Gb, but searching with the web interface has a maximum of 5 seconds (most searches are faster). The one disadvantage is the backup (I use pg_dump once a week which needs about 10 hours). But for now, this is acceptable for me. But I want to look at slony or port everything to a linux machine.