A search bar at the top of the mobile app screen would be ideal, so clients can search for any particular resource they have in mind. A search bar within each sub folder inside Resources would be great too as a bonus, but definitely one in the master Resources area within the app would be amazing.

For example, they are looking for a past lesson, but they can only recall a keyword. Searching keywords to find (and then "favourite") it for future reference would be v helpful.