The content.htm should never be edited offline in an editor, because the assignment of the page data, that is stored in the pagedata.php, maybe will be destroyed.
Maybe, that hidden pages are visible, and visible pages are hidden, after editing one of both files manually.
If you want to work offline, you should install a server environment on your PC, such as Xampp. With this server environment, you are able to operate a reflection of your CMSimple installation on your PC.
Please note, that by data transfer between your PC and the Internet server always both files (content.htm and pagedata.php) have to be transfered together.