If this is what I think it is, the issue is with the PDF display on-screen through the Adobe Reader rather than any plotted line weight. If that is the case you may see a difference in display if you view the same PDF with another viewer. If you are stuck with Adobe Reader, try changing Preferences for such things as "Enhance thin lines", "Smooth line art", etc. (I can't remember the specific toggle off the top of my head).
The most direct solution is to understand what is actually happening (display vs. print), and have others understand as well. Or, try the blunt answer: that's the way it display, it prints fine, no we don't have a way of changing that, spend the time focusing on what *really* matters (ie. the technical content of the drawing).