i know I can put a password in autocad then the autocad need the password to open the file when you want, but my question is: does somebody know if I can put a password only if you want to change something, just to see

This is not possible in plain AutoCAD but you can do this (and much more) with the CAD Vault add-on application.

 
See:
www.xanadu.cz/cadvault  (in Czech)
www.cadlock.com  (homepage)
 

[QUOTE=Apeiron]i know I can put a password in autocad then the autocad need the password to open the file when you want, but my question is: does somebody know if I can put a password only if you want to change something, just to see[/QUOTE]trimite-mi un email te rog.

very goog!