Curves are not smooth in 3D PDF. The lines making up the curves are very visible in the PDF file.
Increasing the Mesh Resolution should improve the curves. If you need accuracy beyond what the Mesh Resolution setting can offer, you can try setting the Mesh Tolerance setting to a small value (1/64 of an inch is the lower limit and produces very fine results, even under close examination). If set to a non-zero value, this overrides the Mesh Resolution setting. The downside to this is that designs with a lot of curves will result in significantly larger PDFs.