A variable volume file for documents, includes a folder, constituted by two flat parts and by a back, and a box, corresponding in shape and volume to the folder, having two larger bases and by three lateral surface faces. The folder and the box are made individually in two separate parts, which can be partly overlapped on one another in the width direction of the back. Each folder part includes a flat and a back wing; each box part includes one of the bases and lateral surface wings. Provided in the back wings of the folder and in the lateral surface wings of the box are structures for sliding engagement which interact with one another for the relative movement of one part in relation to the other and structures for mutual locking of the respective parts in prefixed positions.