Had this problem and Tim Wietzel of Karpinski Engineering was able to help me with this one. What you need to do is:
- Goto your C:\ProgramData\Autodesk\MEP 2010\enu\Plotters directory.
- Click on whatever plotter you use to convert your DWG's to PDF's.
- Once you do that select the Device and Document settings tab.
- Next select the custom properties button.
- Under font handling select capture some.
- Then click the edit font list.
- Once you've done this it will take you to a list of fonts. Make sure ARIAL is checked.
You may note that ARIALMT is checked but the ARIAL font not being checked is what causes this.