In AutoCAD 2004 and up, you can password-protect your DWG and DWF files. When you Save As, you can select Security Options from the Tools menu at top right. 

Caption: Select Security Options from the Tools menu in the Save As dialog box.

This displays the Security Options dialog box, where you can enter a password for the drawing. Check the Encrypt Drawing Properties box if you want to encrypt properties such as title, author, subject, and keywords.

Caption: Security Options dialog box.

Select the Advanced Options button to select an encryption provider (options here will depend on your operating system) and key length (the longer the key length, the greater the security).

Caption: Advanced Options dialog box.

You can also set a password via the Options dialog box. Select the Open and Save tab, then click on the Security Options buttons in the lower left corner.

Caption: You can also set a password through the Options dialog box.

One final option for setting the password is the direct route from the command line. Just type in Securityoptions.
Keep in mind that if you lose or forget the password, there’s no way to retrieve it. Always keep an unencrypted copy of any password-protected drawing in a safe location.

There are many things that AutoCAD for the Mac can’t yet do. Even if you are using AutoCAD for Windows, you should be aware of the Mac limitations in the event you have to exchange files with someone who is using AutoCAD for the Mac. Take the previous tip, for example. AutoCAD for the Mac cannot currently open password-protected files. There’s no ability to edit dynamic blocks, and no support for hyperlinks. Multilines have only limited editing and style support. There may be other features that don’t survive the trip across operating systems. If at all possible, keep your drawings on the operating system in which they were created.

Another omission in AutoCAD for Mac is the ability to create and edit CTB files (plot style tables). AutoCAD for Mac still uses plot styles when you plot, but there is no Plot Style Manager with which to change them. One workaround, described on the MacACAD site, is to use the free DraftSight program from Dassault/SolidWorks, which does support CTB files.
AutoCAD stores its CTB files in a folder called Plot Styles. Use the Finder to search for this folder. Or start the Print command in AutoCAD (File menu). Select the Advanced button. In the Print-Advanced dialog box that appears, open the drop-down list for Plot Style Table Name and select Reveal in Finder.

If you want to edit an existing AutoCAD for Mac file, copy it to DraftSight’s Print Styles folder:

/Library/Preferences/DraftSight/1.2.119/Print Styles/

Alternatively, you could start with one of DraftSight’s CTB files, make your changes, then Save As to AutoCAD’s Plot Styles folder

Download DraftSight from http://www.3ds.com/products/draftsight/draftsight-overview/ and install it on your Mac. Select Print from the File menu. In the Print dialog box, select Print Style in the drop-down list beneath the Orientation section.

Caption: Select Print Style from the pull-down menu below the horizontal rule in DraftSight’s Print dialog box.

Next, select the CTB table you want to edit from the pull-down list under Print Style Table.

Caption: Select the style you want to edit from the Print Style Table list. It will display any CTB style you’ve stored in the DraftSight PrintStyles folder.

Then select Edit in the Print Style Table section. Click on New to create a new CTB file. When you’re done editing the plot style files, save them back to the AutoCAD Plot Styles folder.

Another handy security feature is a watermark. Unlike Adobe Acrobat, for example, AutoCAD does not have a watermark feature. You can create your own watermarks by creating a new blank layer in your drawing. Select a suitable color--light gray, for example--and create the mtext for the watermark. Alternatively, you could set its pen color to a light gray or give it a low screen percentage. Lock the layer once you’re done.

Caption: Do-it-yourself watermark.

If you intend to use the same watermark repeatedly, you may wish to convert it into a block using the Bmake command.

If Help is no help, you can directly access the AutoCAD Services and Support Web site by typing the command AI_Product_Support at the command line. Your browser will open to http://www.autodesk.com/autocad-support. There you can search the knowledge base for solutions to your problem. According to Autodesk, there's a number of variations on this command. AI_Product_Support_Safe redirects you to a local HTML page if you don't have an Internet connection. AI_Custom_Safe brings up the Autodesk Developer Center. AI_Training_Safe summons the Training page. And AI_Send_Feedback displays the Autodesk Contact Us page.

If you lose a toolbar, type -toolbar at the command line. Press Enter, then type in the name of the missing toolbar. Press Enter again, then select Show (followed by another Enter). This will restore your missing toolbar to the AutoCAD screen.