At this time, the only way to change the image format is by deleting the existing article and then creating a new one. When you create a new article, you specify the image format.
PDF format page can be viewed in the Content Viewer. You can pinch and zoom on any page that does not have any interactive overlays. At this time, there is no searching/selecting text.
The desktop content viewer on my computer can not display the folio, when a page is set to pdf format, it comes up with Error this folio contains one or more stacks that not yet is supported "pdf and flattened stacks" is only supported on iOS devices