FOP is the world's first print formatter driven by XSL formatting
objects. It is a Java 1.1 application that reads a formatting object
tree and then turns it into a PDF document. The formatting object
tree, can be in the form of an XML document (output by an XSLT engine
like XT or Xalan) or can be passed in memory as a DOM Document or (in
the case of XT) SAX events.