Where to start if you need to develop on the iText library itself.
First, you’ll have to check out the source code from subversion. The svn root holds a couple of things so for itext itself do
svn co https://itext.svn.sourceforge.net/svnroot/itext/trunk/itext itext
this will checkout the itext core trunk. If you’d like geospatial and richmedia packages you also need to checkout xtra trunk
svn co https://itext.svn.sourceforge.net/svnroot/itext/trunk/xtra xtra