SFTPS is based around a concept of centralized document storage, limited access, and change tracking.
Centralized Document Storage:
Any decent Secure File Transfer Protocol Software(SFTPS), whether it is open source or commercial, will allow documents to be stored in some sort of centralized location. This makes finding documents in the SFTPS much easier, as there is only one place to look.
Limiting access to the SFTPS files is a good step in the right direction but does no good if the files can be changed, added, removed without tracking of those actions. A good document management software system will add on change tracking for the files so that changes can be noted.
Having a centralized location to store files is not enough to call your product a document management system. A simple folder on your computer can allow files to be stored,but limiting access to specific individuals becomes more of a problem. A good SFTPS will allow fine-grained access to each and every file.