From version 2.0 the plugin support hierarchical category. With this you may organize your PDFs well.
You may create a category structure as the following:
Category 1
----Category 1 - 1
--------Category 1 - 1 - 1
----Category 1 - 2
----Category 1 - 3
--------Category 1 - 3 - 1
--------Category 1 - 3 - 2
Category 2
Category 3
The max depth of categories is 3 but no limit to sub categories count.
When you create category, there's a field of Parent Category you just need to select right category.
