Download and Install: MeshLab & FreeCAD
(optional free CAD packages: Netfabb & Meshmixer)

Obtain a watertight 3D Polygonal Mesh File!

MeshLab
Menu bar:
Filters > Remeshing, Simplification, and Reconstruction > Quadratic Edge Collapse Decimation
Set: “Target Number of Faces” below 10,000 (this makes it easier for FreeCAD)

FreeCAD
with file selected (from the left side file tree)
change the workbench drop-down menu to : Part
then select from the Menu bar: Part > Create Shape from Mesh... Part> Convert to Solid

Troubleshooting:
If your file consists of several overlapping objects you may need weld or remove the interior faces together using Autodesk's Meshmixer by using the “Make Solid” feature.

If your 3d Mesh File is not water tight, you may need to fix/repair it. File Resizing and Scaling issues are also prevalent. These options are easy to do in Netfabb, directly