When you say most people or most books you are speaking in general terms about the majority of people (in a country, in the world) and the majority of books (eg. are soft-back or are now available online).
Most people would agree with me .....
Most people would object to your views...
When you speak of most of the people you are talking about people in a particular context and not universally:
Most of the people in the hall....
Most of the people who went to university...
The same applies to books:
Most of the books in the library....
Most of the books that I have read...
Thus most of the documents would refer to specific documents
Most documents would refer to documents in general.