I think you got it backwards. A relational schema change is required and probably a data reindex or reload when you change a data model so relational is the least flexible as opposed to something like Mongo nosql?
If you’re just talking about storing and retrieving documents in a hierarchy with a semi-fixed format then aren’t you asking for a filesystem?
How do you propose that you query a filesystem? In this context, we are referring to a “document” as a structured JSON document. You still have a semi known structure with fields that you can index and query.
A schema change can handle changes needed to existing data, if you don't do that then sure Mongo is easier, but then you've broken data in your system.