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
Mario - GemBox
Hi,
This feature request has been implemented and is available in the latest versions of GemBox.Document.
Regards,
Mario