If you don't have access to a folder then you will not see any documents living in that folder when doing a search.

For example User A has access to the folder with documents and they do a search, they should pick up those documents in the results. If User B does a search and doesn’t have access to the folder with the documents they will not pick up the documents in the results.

So in the example mentioned above, User A will see the documents but User B will not see the documents.

You can't apply security access to a document. It inherits the security applied to the folder in which it lives. So, if you have read only access to a folder then you are assumed to have read only access to any documents in that folder. If you have read/write access to a folder then you will be able to read as well as upload any documents to that folder.