FOP is another project from the Apache Software Foundation (more info at http://xml.apache.org/fop), a print formatter driven by XSL formatting objects.
In Cocoon, the FOPSerializer component uses FOP to generate PDF, PCL or Postscript documents from an XSL-FO document created by the Generators and Transformers that come before it in the Pipeline.
The HTML and PDF publishing How-To gives a minimal example of HTML and PDF publishing.
FOPPDFEncryption explains how to create encrypted PDFs
FOP can be very memory hungry, your milleage will vary but a brief FOPTuningGuide may help you to tune your system.