it is error everytime when i try to open download zip file.
You mean downloaded from GrabCAD or every file?
Sure it's a zip file, and not a rar file. It maybe corrupt, or your using and old version of winzip or winrar. Winzip and Winrar will open each others files usually.
I'm having a similar issue, whichever file I've downloaded is showing an error. I even tried opening the files on two separate computers.