Depending on the browser I have the possibility to clip the file as txt, RTF or in markdown (Chrome) or as bookmark, picture or markdown (Safari). what am I missing?
See the process described here to export a PDF from Safari to DEVONthink to Go (and other apps)
This is a better set of instructions:
DTTG’s Clip and Import extensions do not support PDF clipping. (Very few extensions for any other app support it.)