We recommend the following steps to improve the image quality:
  1. Go to Printers and Devices
  2. Right click the PDF printer, Printer Properties
  3. Click on the Advance tab
  4. Click the Printing Defaults button
  5. Change the resolution to 2400 or 1200 or whatever the highest resolution that is available
  6. Click OK/Save/Apply 
The image quality may still look poor when you preview it, but it should print with high resolution.