How to let non-Dropboxer upload file or look at file from a particular 'Directory' using Ruby on Rails?

This certainly sounds possible. The API was designed with the intention that each user would link their own Dropbox account, in order to interact with their own files. However, it is technically possible to connect to just one account. The SDKs don't offer explicit support for it and we don't recommend doing so, for various technical and security reasons. However, most of these security concerns are allayed for web apps where you can keep the access token server-side.

So if you did want to go this route, instead of kicking off the authorization flow, you would manually use an existing access token for your app. (Just be careful not to revoke it, e.g. via https://www.dropbox.com/account/security .)

You can upload files using the Dropbox API via your web app, e.g., using the get_file method in the official Ruby SDK:

https://dropbox.github.io/dropbox-sdk-ruby/api-docs/v1.6.4/DropboxClient.html#method-i-get_file

You can upload files using using the put_file method:

https://dropbox.github.io/dropbox-sdk-ruby/api-docs/v1.6.4/DropboxClient.html#method-i-put_file

You can even upload files to subfolders that don't exist yet and the subfolders will be created automatically.