Sometimes the number of sections in a menu referring to a document is more than can be reasonably handled in a simple dropdown menu. In that case we have the problem of making sub sub menu items. These are complicated to program and still do not conserve page space very well. The present solution is to program collapsible sections that refer to smaller sub-sections.
Think of a book that has many sections each of which has many chapters. To examine one section, we expand it so we can see its chapters. When finished with that section we contract it and go on to the next section. Having finished with that book we press its top navigation menu item and the whole book menu collapses. Then we go on to the next book.