Currently GemBox.Document converts vector images (WMF and EMF formats) into a PNG format when exporting to PDF file.
GemBox.Document does provides a MetafileScaleFactor property with which you can improve the quality of converted PNGs, however it is still a lossy format resulting in larger pdf size and less quality than vector images.
Official response
Hi,
This feature request has been implemented and is available in the latest versions of GemBox.Document.
Regards,
Mario