NetFabb fixes mesh error, closes holes, and attempts to make the object manifold so it can be sliced and printed without errors.
However it doesn't do anything to modify or simplify the object. The Pro version has a "Refine Mesh" option, which appears to do a triangle reduction, but I can't verify that since I don't have the Pro version of the NetFabb.
Again, this is what MeshLab does best. It has the means to simplify the mesh. So if you need to reduce the number of traingles in a mesh, load the STL file in MeshLab, then from the filter menu select Remeshing, Simplification & Reconstruction / Quadratic Edge Collapse. from the PopUp menu select the desired number of face (by default it's half the current number), and click apply. When done, export your mesh as STL and you're done. Simple, no complications.
FWIW, NetFabb is intended to clean up the mesh to get it ready for printing. Whatever edits you do afterward, will most likely break the fixes NetFabb applied, so use NetFabb only as the last step before printing the object.
To print or, 3D print, that is the question...
SD3 printer w/too many mods, Printrbot Simple Maker Ed., FormLabs Form 1+
AnyCubic Photon, Shining 3D EinScan-S & Atlas 3D scanners...
...and too much time on my hands.