First off, do you have access to the underlying database? It is always easier to query the index data directly if that is the case.
If you don;t have access to the database, then you need to get back document information one of two ways: 1) Pull down all documents in a file cabinet, or 2) Execute a query via the SDK to get a list of documents back. When you get a "Document" object back it isn't the actual document, but the record representing the indexes for the document (which is what you need).
This describes how you get all documents from a document tray or file cabinet:
and this explains how to execute a query:
Note that the data you get back from this is not as easy to parse as, say, a relational database table. But you can parse the returned documents using loops and things like LINQ to pluck out the information you need.
Like I said, accessing the data directly is much more straightforward, so if that is an option, forego the SDK and hit the table directly. Just don;t do any modifications via that methodology.