Hello Max99,
yes you can without any conversion in Calibre, just on your DXG - but not with the Kindle Operation System.
The idea is to load the Duokan OS onto your Kindle. With Duokan you can darken/zoom/crop your PDFs. You will have dual-boot possibility (Kindle OS remains as is and additionally there is the Duokan OS on your DXG). See this thread for details:
http://flip.netzbeben.de/2010/11/duo...ish-gui-today/
I also have tons of pdf on my DXG and I read them only in Duokan while I do the web browsing in Kindle OS.
Best wishes
Biodu