I've encountered this problem too. I recently did some digging. I DON'T know if it applies to your situation, but maybe this clue will help. In the PLOT dialog on the far right side is a quality pull-down arrow. I selected the maximum quality setting and got the improvement I needed.
THEN based on this clue, In other programs like WORD and a couple of others I use for page making when arranging extracted images of my 3D Renders, etc, I looked for the corresponding settings in those PRINT dialogs under ADVANCED tabs, and the like, and was able to achieve consistently acceptable results.
I don't know if this will help you, as I'm running 32-bit ACAD 2011, but maybe so. My PDF quality now appears on screen and at the printer within acceptable quality limits, and I get even better results when I shift to one of the 2 other PDF generators I use frequently.
Good luck, and post back here with results ?
Tks-
C. Fee