You received a too complex and detailed 3D model as a surface mesh and now you need to simplify it before using it for 3D printing, 3D rendering or converting it to a solid 3D CAD model.
An ideal tool for processing very large 3D mesh models is Autodesk Memento (Autodesk Labs, memento.autodesk.com). This free (status: 1/2015) application allows to simplify (decimate) a model without changing its shape, to solve topology problems (holes, overlaps, spikes), to retopologize (re-mesh) the model, set its scale, 3D print it and perform other operations.
You can import 3D mesh models in the formats STL, PLY, OBJ and RCM. Processed meshes can be exported to the formats OBJ, FBX, STL, RCP, PLY.