This one should be a FAQ. It comes up all the time.
I have a similar issue with AutoCAD LT 2009. Some text also appears to be BOLD and fuzzy, in addition to the messed up line weights.
Go to AutoDesk website and download the FREE TrueView 2010 or 2011 whichever is available. Open your drawing with TrueView and plot as usual. It does a fine job of plotting pdf files.
TrueView can also convert newer drawing versions to older ones so your AutoCAD 2008 can open them.
Did I mention it was FREE?