LibreOffice intermediate representation before exporting to PDF

Hello, guys!

I’m interesting in subject question: What is the software intermediate representation of LibreOffice Writer OOXML document for example, before it is written by some export filter to PDF? Is it looks like XML-tree or JSON property:value pairs?! I’ve look at the source code of LO, but found only some abstract UNO interfaces, so please help me with my research.