We would need more information.
Are objects with transparency nearby those letters?
Are you sure, that the font is not bold? Did you check nested styles or Grep Styles?
No, no objects with transparency, font is not in bold, no nested style or grep styles. No idea why it's doing it.
I had the same problem but with lines of text. It was due to ligatures.
from Charles_T._Mills: "The random bolding problem is caused due to Acrobat not handling
ligatures correctly. Ligatures are special characters created
automatically for letters like "f" in relationship with other letters.
The problem must be fixed in InDesign by turning off "Ligatures." To
do this select all the text in a text box, select the "Character" tab,
and then select the character options menu in the upper right hand
corner of the dialog box (small triangle and three lines). In this
menu uncheck the check mark next to "Ligatures."
You must repeat these steps for all text boxes. Save the file and then
re-export a new PDF. This will fix the problem. Of course you can't
use ligature characters...but that will be something Adobve will need
to fix in Reader. "