You probably can't, not using iText. That's simply not a tool made for extracting layout information from a PDF. I'm not aware of an open source or free Java library you could use for this, but PDF-Renderer can display PDFs in Swing, so obviously it knows how to get at layout information.
There are 29 Knuts in one Sickle, and 17 Sickles make up a Galleon. 42 tiny ads in a knut:
a bit of art, as a gift, that will fit in a stocking