Just search Java examples?

No, iText used with Processing.
I have experience including in Java and jsp but I did notfind the start with Processing.

See  http://www.processing.org/reference/libraries/pdf/index.html

The question was to use iText version 5.x
thanks

Where is your problem? Processing PDF export exactly uses iText.

But processing uses an early version (2.1) , which cannot make for axample PDF/X3 and embedded fonts.
The new version is not under the GNU license. So you have to buy it for professional use.

If you have a commercial license (you can also use the free license for trying out first) , I'm pretty sure you can bind in the .jar file as one can do with other Java libraries as well. 

Just put it into the libraries folder in your Processing sketchbook folder, e.g. ~/Documents/Processing/libraries/itext/library/itext.jar

Example files for iText itself are available on  http://itextpdf.com/examples/

You may also look at  http://code.google.com/p/processing/source/browse/trunk/processing/java/libraries/pdf/src/processing/pdf/PGraphicsPDF.java to see how Processing interfaces to iText. Interesting comment there: 

1. // Tried iText 5, but it was too slow

   //import com.itextpdf.text.*;

   //import com.itextpdf.text.pdf.*;

" No, iText used with Processing." That's not version 5...
" The question was to use iText version 5.x" Precisely.

I pointed to Java examples, because Processing is just Java, with some syntax sugar. But Java code is easy to adapt.
If your problem was how to use such library, mbraendle answered precisely. Don't forget to rename the jar if needed.