What settings are you doing the pdf at, if its a low setting the image will look poor, if the pdf settings are really uncompressed then the image will look better, also pdfs dont display vector artwork very well (depends on what version you are using) is that image vector or bitmap?
Hi I have just tried an image making a pdf from Photoshop, and set at 12 (max) with image interpolation ticked, and it looks fine. is the image that you are pdfing compressed already, if so try getting the image with no compression and pdf that one (or pm me a link to the image and I'll have a look at it for you)