Knowing exactly how you create them (including versions of OS and programs)
would help to help you...
I am using Windows 7 64 bit, AutoCAD 2010 and Reader 9. Iv tried many fonts but the fine lines appear faded.
Still you haven't metioned how they were created. Are you using an Adobe product to create the PDF or is it a feature of AutoCAD? As many details as you can give would be helpful. If the latter, you'll probably want to ask in an AutoCAD forum.