Attach the file here, It seems that the file is created in any higher version and you are trying to open it in a lower version. I have both V5R20 &V5R25, Let me try to open it for you.
If size is larger and you are unable to send by email. Use Winrar to compress the file and see if you can send it by mail. If not send it to dropbox and forward the link ensure the file is password protected and can be opened by you and your friend only. Companies send the large CAD files through FTP link which need licensed services.