PDF processing

Hi all,

I am quite curious about this fantastic feature of LibreOffice which
allows me to open a PDF and edit it in the UI - even though it seems
to screw the fonts a bit. I wanted to have a look at the source code
to see if there is some sort of PDF "model" being built from the
original PDF document, for instance a  set of objects each describing
the graphic meanings of a particular region within the page.  I did
not succeed very much as I am not familiar with the codebase and the
project is quite big.

Can anybody point me in the right direction ? Is there a specific
place where this PDF "normalization" takes place ?

Thanks,
Pietro.