how can I convert dwg files to pdf using system 10.9?

Use AutoCAD, and Print the file. In the Print dialog there is a menu item to save to PDF.

Other DWG compatible applications are mentioned on http://www.architosh.com/ and my tip:

kmosx: Can I run AutoCad on Mac OS X?

Edit:

Pretty much all Mac OS X programs support printing to PDF.

The quality of the PDF may not be as good as one with Adobe Acrobat Pro installed. If you have Adobe Acrobat Pro installed you may be able to customize its quality better.

I followed the 'print, then save to PDF’ steps....but then the quality was not good.

What has worked for me is to follow same step....’print then once the print dialog opens you say ‘open in preview'.

When the drawing opens in preview you just save it to your desired location....

OS X Yosemite 10.10.4