iText is a free/open source software (F/OSS) project, which lets you read through the full-code API to better assist you in creating a solution. F/OSS does not mean that it is free of obligations, it just means that the code is freely available. By following the rules of the Affero General Public License (AGPL), you may use iText at no cost. Otherwise, it is mandatory to purchase a commercial license to remove the copyleft restrictions.
The AGPL license comes with the following restrictions. The points below are not exhaustive, but merely the most important restrictions:
You may not deploy it on a network without disclosing the full source code of your own applications under the AGPL license.
You must distribute all source code, including your own product and web-based applications.
You must disclose any modifications made to iText.
You must prominently mention iText and include the iText copyright and AGPL license in output file metadata
If you look into the source code of iText, you'll see that most files contain a header referring to the AGPL. You should have received a copy of the GNU Affero General Public License along with this program; if not, visit the GNU license pages. You must retain the producer line in every PDF that is created or manipulated using iText. You can be released from the requirements of the license by purchasing a commercial license. Buying such a license is mandatory as soon as you develop commercial activities involving the iText software without disclosing the source code of your own applications. These activities include: offering paid services to customers as an ASP, serving PDFs on the fly in a web application, shipping iText with a closed source product.