I might be wrong but processing comes with itext.jar which correspond to iText 2.1.7.

I think if you add :

1. import com.lowagie.text.*;
   
2. import com.lowagie.text.pdf.*;

You might be able to use itext 2.1.7 core functionality.

If you need iText 5, you can download itext.jar and add it the your current sketch by drag and drop create a code folder in your sketch's folder and add required jar files. Then using correct import statement you can use core iText classes.

From processing sources, devs have reported that iText v5 have an issue:

The issue is that versions from the 5.x series were slow to handle lots of
 * fonts with the DefaultFontMapper. 

Note that if you use iText 5, your source code must be GPL, a restriction that v.2 doesn't have (it is LGPL IIRC). And I guess that for most frequent PDF generation purposes (beside forms, scripting, etc.), v.2 is more than enough.