Documentation
API documentation
For every new release the API documentation is automatically generated using javadoc. You can consult the API online or download a tar.gz file with all the API documentation.
Tutorial and Code Samples
There's a very comprehensive tutorial (iText by Example) under construction, with hundreds of small standalone examples. Just puzzle together these code bits to build your own applications. Another good place to start, is the examples-xxx.zip available on Paulo's Early Access page (replace the xxx with the current early access release number).
Mailing-list
iText has a very active mailing-list. If you don't want to register rightaway, you can browse through the mailing-list archives before sending your question to itext-questions@lists.sourceforge.net. Try describing your problem as concrete as possible. We can't answer questions such as "Why doesn't iText work on my computer". Please keep in mind that it is really difficult for us to solve problems or track down bugs that we can't reproduce. If possible, send us a small standalone source code example, demonstrating the problem. Make sure we will be able to compile and execute it on our own virtual machine (we probably don't have the same database systems, external libraries, application servers,... you are using). Also send us the resulting PDF on your machine, so that we can compare it with the PDF that is generated by us, using your code.
Third Party Presentations
If you have written a presentation, a tutorial or some code samples you want to share, please let me know and I'll add them here.

There's an article on AdobePress about iText written by Jeff Friesen; it's titled Tools of the Trade, Part 1: Creating PDF documents with iText. You can also read the step by step guide Generate PDF files from Java applications dynamically by Amit Tuli (IBM). If you want to start with a very simple example, Christoph Bartneck has written you some pages on the step-by-step installation for Windows and Mac. There is also a page on Creating PDF-files.
If you need more and if you are used to a Web Application environment, Mark Stark has made a powerpoint presentation and a war file, introducing the generation of PDFs, filling/reading Acrobat forms and deploying iText in a web application.
Another interesting presentation is the powerpoint Sean Sullivan made for the Portland Java Users Group in Portland Oregon USA. The topic was iText and PDF. You can download his PowerPoint slides here or you can read the article he wrote for O'Reilly's onJava.com: Dynamically Creating PDFs in a Web Application.
If you want to fill forms, have a look at this blog-entry by Vince Schuurman.

For an introduction in German, I can refer you to an article in JavaMagazin by Cord Jastram. In a few selected examples, it gives you an overview of some powerful features such as generating an encrypted document, using PageEvents, generating Graphics, etc...
Ilie Nae wrote a similar article in Romanian for NetReport: Crearea de documente PDF in JAVA
If you understand Portuguese and you are using iText in the context of a Web Application, you can read the article Como fazer um servlet gerador de relatorios PDF by Pedro Burglin at Cafeh.com.br.

There are some specialized examples around too: The Serverside has an article on Inserting text to existing PDF file. There are examples available on making charts with JFreeChart and exporting them to PDF. If you want to use iText in a ColdFusion application, just read blog example first.
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