1. A document management system that defines an index type for each management unit that is a unit for managing a plurality of documents, the document management system comprising: a search area selection unit configured to select one or a plurality of management units as a search area; an index acquisition area selection unit configured to select one or the plurality of management units, from which the index type is acquired, as an index acquisition area; a search condition setting unit configured to display in a list the index types defined for the management unit in the index acquisition area selected by the index acquisition area selection unit, make a user select a desired index type from among the types of the indexes displayed in a list, and set a search condition using the selected index type; and a search unit configured to search for a document corresponding to the search condition set by the search condition setting unit from among a plurality of documents managed by the management unit belonging to the search area selected by the search area selection unit.