I am googling to see if you can set the resolution of the PDF file (set the dpi in CFDOCUMENT), but I don't see that anywhere. Seems like you cannot set the resolution.
I also read that it's optimal for your image to be 96 dpi for cfdocument. That was just one person's observation.
My recommendation would be to lower the resolution of your image due to the limitation of cfdocument; it seems it cannot be set at the higher res.