I am the CEO of the iText Group, the original author of iText as well as the author of two iText in Action books and the free ebook The Best iText Questions on StackOverflow.
The list you refer to mentions iText and with this answer, I want to confirm that iText still exists. It is available under an open source license (AGPL) as well as under a commercial license (which generates revenue that is used for further development).
Apart from being the CEO of the iText companies (with offices in Europe, the US and soon also in Singapore), I am also a member of the ISO committees that create the PDF standards. In less than a week, I am flying to San Jose (CA) to attend the meetings that will discuss ISO-32000-2 (the upcoming PDF 2.0 spec), ISO-19005 (PDF/A), ISO-14289 (PDF/UA),... I am also responsible for the ISO Adhoc committee for digital signatures (rewriting the part about digital signatures in ISO-32000-2).
At iText, we invest in further development, which isn't always the case for open source libraries that are offered with a free as in free beer license. For instance: both iText and PDFBox are a member of the PDF Association, but at the PDF Days in Cologne in 2014, PDFBox declared that they had no resources to invest in support for PDF 2.0. The project stands or falls with the existence of contributors who donate code. The business model used by iText generates money that allows us to pay developers.
Flying Saucer is a project that is built on top of iText, but it is not affiliated with the iText Group in any way. I don't think it is still supported, but I am not sure.
I didn't know about PDFClown until the developer of the tool teased me about a single feature that was available in PDFClown, but not in iText. I responded by implementing that feature myself. Based on the conversation, I assume that PDFClown is a one-man project.
I don't have much information on the other projects. One project that seems to be missing in the original list (but present in the comments and in another answer), is Apache FOP. Based on a survey that was organized by Black Duck Software on our behalf, Apache FOP seems to be running on many systems (it was the #2 after iText), but not that many developers were happy with it because of (1) the architecture that depends on XSL-FO, and (2) performance issues.