DXF Reader
Many file formats don't have explicit information about polygon normals. Some
use the implicit method that derives a normal from a perpindicular between two
edges of a polygon, however, some programs use different sets of edges to
determine that normal. (For example, one program might use the first two edges
in a polygon, another might use the first edge and the last edge.) Another
problem with programs that export to DXF is that they need dto be very careful
when turning a multi-sided polygon into triangles or quads for DXF's 3DFACE
entities, because if they aren't careful about normals, you'll see each polygon
turn to "swiss cheese" in destination 3D programs that are sensitive to
normals.
My company makes InterChange, a program that translates between many different
3D file formats. If you have any translation questions, I'd be glad to help.