Maybe this product can help you: http://www.n2pdf.com. It allows to create PDF files from Notes documents. based on script integration. It works on client and server.
I don’t know iText, but what definitely works fine is to use XML in combination with XSL-FO and the Apache FOP to create PDFs. The Apache FOP is an open source Java implementation. There are several examples onhttp://xml.apache.org/fop/
It is much complex than you can think about. There are other responses quoting some XSL:FO transformers for converting the DXL into PDF. However, the crux is you have to create a stylesheet to instruct those XSL:FO transformer to perform the conversion. Depending the complexity of your documents (e.g. involve RTitem or not), the “stylesheet” can drain up weeks (or even months) of your time to devleop. You may even have to, in addition using the stylesheet, write some XML program to “massage” the DXL file to make it become renderable into PDF. Take a look on the stylesheet to convert the Notes document into HTML. You will see my point then.
I think people in IBM/Lotus may also know this limitation but they have no incentive to solve it because that will probably endanger the life of the properitary CD record format of Notes database.