CAD tip # 5666:

But you can use a VisualLISP utility to open R/O DWG drawings without this message. You can even redefine your standard OPEN command:

(vl-load-com)
(command "._UNDEFINE" "_OPEN")
(defun C:OPEN ( / fn)
 (if (setq fn (getfiled "Open R/O" "" "DWG" 0))
  (vla-activate (vla-open (vla-get-documents (vlax-get-acad-object)) fn :VLAX-TRUE))
 )
)
(princ "\nCommand OPEN redefined, use _REDEFINE to revert")(prin1)