The ability to search a magazine/app via a text input field would be a useful addition to DPS. Naturally, the TOC currently provides the user with a method to navigate or drill down within the content - however, the ability to search for a specific term, in my opinion, would improve the user experience.
Whether it is possible to retain the ability to search text on a page if articles have been created using the PDF format - I don't know. When using PDF format, are text elements retained? Or is a flattened image rendered at some point?
Alternatively, or in addition, the search could use the TOC and article properties as a basis for its functionality.
Yes, I like the idea of the viewer framework and interface expanding to include new functionality that doesn't have to be hard-wired into the folio pages. Search is a great example as the viewer app would need to have some sort of consistent format for displaying search results and allowing the user to scroll through them. Likewise, pop-ups for displaying contextual information on folio pages would be greatly appreciated. This kind of approach is where I think Apple's iBooks author shines. You provide assets in the form of text, images, metadata, etc. and the interface sorts and displays them for you.