From the Training Materials page, under the heading In the Category 7 categories can be chosen from. Select the category that you'd like to filter by, then hit Apply Filter right beneath the categories. The results will show you all files in the category you have filtered by in the middle activity feed section. 
You can also search for files at the top of the screen by putting in a keyword in the search bar.
To clear your search, click the x that’s located in the search bar.