I agree with others that PDF is a great format to use to communicate with associates and clients. It is a natural output of DataCAD and users need little computer savvy and no CAD. But I also see the need to write on those pdf's with some sort of editibility so comments can be made and returned to sender for evaluation and/or changes. I also agree that it is not smart to send cad files just so people on the other end can see our work and add notes, like some have said here, they'd need to buy DataCAD and learn it. Even the light version at $300 isn't quite "free". I did a Google search to "redline pdf" and came across Bluebeam PDF Revu at
www.bluebeam.com. You can download a fully functional program that will work for 30 days. The standard edition is only $150, and has a wide variety of markup tools. I downloaded the 30 Meg program in minutes, it asked to download a 22 meg program (.net something) and was up and running in less than 10 minutes. In a few more seconds I had added a sticky note, and had written a note directly on the drawing, and had added a balloon, with my choice of line width, color, opacity and fill color and opacity. Pretty nice. This could be the program we've all been wanting. If you click on the link, you can download a nice big image showing those markups in a screenshot of the program window.
http://www.orbitfiles.com/download/id1012983490