Hello Pascal,
l upload a picture of the line widths and linetyps settings of rhino 5 and 6. Also I updated all drivers of my computer and used different pdf-printers like bullzip, cute pdf, adobe pdf, rhino pdf etc. In Rhino 6 I get all the time the same line width problem. 
My system is a alienware 17 a4 including a nvidia 1080gtx graphic card and with a 4K monitor.
Until now I am very happy with Rhino 6 except of this vector printing problem…
