Document index is a view to see all folders and documents in Boardeaser. The advantage of using the document index is that you easily get an overall view of which documents are under which folders. It is practical to use the document index if you have an extensive collection with many folders and documents.
Click "Documents" and "All documents" in the left menu. Then click the icon with three dots (···) and select "Document index".
This is the Document Index view. From here, you can open folders and documents as you do in the regular "All documents" view.