Reading External Documents

Hi!

I was wondering where to begin on reading file types that are not your own. For example, screenwriting applications seem to be able to add support for Celtx and Final Draft documents. Another example are the storage applications which seem to be able to support iWork documents.

I've tried searching, but it is difficult to search for this kind of topic because the search results just point to tutorials showing how to put files in existing applications, how to make your own file type, and AppStore links to applications for reading PDFs or something.

Thanks!