Try turning off "enhance thin lines" in the Page Display area of the preferences.
-- and do not trust your screen. If the line is 0.5 pt thick in InDesign, that's what it is in the PDF, regardless of how much you zoom in or out.
Will sent me a PM because he's having trouble replying in the forum:
Hi, thanks.... but that is a setting in Acrobat.
The designer that I work with says that her InDesign doesn't have this line weight problem (or she has never seen this problem before), so is there a setting that I can change in inDesign?
(I tried to reply on the forum but I keep getting an error... 'You can't post a blank message')
This is not a problem in InDesign, it's a problem in the display in Acrobat (and you are the second person today to post about it). Acrobat is the place you need to be to fix it when viewing the PDF.
As far as your posting problem, are you using the web page editor, or email? Some email clients have issues. If the web page, you need a little patience sometimes for the editor screen to fully load, and then you may need to click inside the editing area to get an active cursor. Hitting Enter with a blank message area would generate that error.
I have looked over 4 forums on underline thickness issues and finally found this solution. It worked. Thank you!