Even when connected to FREE HubSpot account, it is possible to upload and retrieve Files API. Since for these free accounts, the Marketing > Files module is not available, an account cannot see the file uploaded via API.
The question is, why these files are not visible under some other page (i.e. Sales > Documents), and is there API support for the Sales > Documents?
Hi, @AKaurin 
Thanks for the questions.
For Q1 — one use case for the Files API and Free accounts is the ability to upload a file and associate it to an Object. I have a working example with steps (using Postman) here — Attach file to (an object) record using API.
For Q2 — There is not an available endpoint to upload or interact with Sales Documents. If you have a moment, adding your suggestion to the Ideas Community and getting some upvotes will be very helpful in getting your suggestion in front of the Product team. If you add a link back to your Idea post, I’ll be sure to give it an upvote to help get the ball rolling.
Best,
Jaycee