Problem: If you open a file that was edited by hand and run into this error: "This file contains a malformed page deck. If the file was edited by hand, please make sure there is exactly one page deck containing only pages as children, and make sure each page has a single child with the class gwd-page-content"
Solution: open the html in a text editor and make sure the elements are in the gwd-page-content div.
Example of malformed page deck in a banner ad:
<div is="gwd-pagedeck" class="gwd-page-container" id="pagedeck">
<div is="gwd-page" id="page1" class="gwd-page-wrapper gwd-page-size gwd-lightbox" data-gwd-width="300px" data-gwd-height="250px">
<div class="gwd-page-content gwd-page-size"></div>
<div>
This element is causing the issue because it's outside of the element with the class gwd-page-content
</div>
</div>
</div>