Hi gaaldornik, it's the self-intersection areas that MO illustrates above that are the problem. Spots like that where a curve (or surface constructed from such a curve) kind of backtracks and squiggles over top of itself will make surface intersection and volume analysis algorithms not work correctly.

Data in PDF can often have this kind of messy geometry that needs to be rebuilt before being used for 3D construction. When the curves are only being used for 2D imaging it's not as sensitive to the messy areas as 3D solids boolean calculations are.

- Michael