Hi letric,
If you right click on the drawing dwg before you open it, select properties, and then check "read only" then un-check everything else, then the file is "read only". You cannot use the "save as" function to make changes unlesss you rename the file. If you try to make changes, without renaming the file first, you will get a message that says "write protected" and you cant change anyting. As I mentioned, this is just to keep someone from accidently making changes. It's not a "security" option.
Another option is to simply lock all drawing layers, and then nothing can be modified, but then you can really only view the drawing, if you take meaurements and want to delete the measurements, then you will have to unlock the layer.
Are you afraid someone is going to accidently make changes and just screw up the drawing, or is it security related where you are afraid someone will modify your DWG and use it for their own profit without your permission ?
John