Dtk_API is a singleton.
You have to enable the readers you want to use and then start the API. When API is started you can Use Dtk_API::OpenDocument() method to read a file.
This method recognizes file format and calls corresponding Dtk_Reader. You have to close the opened document with Dtk_API::EndDocument() method to free data used by the reader. At the end, you need to stop the API.