What XSL FO processor do you recommend for conversion to PDF?

The most popular XSL FO processors to PDF are FOP (http://xml.apache.org/fop/), XEP (http://www.renderx.com/), and Antenna House XSL Formatter (http://www.antennahouse.com/XSL20/PDFoption.htm). The FOP is an open source project. At the moment, it is not a full implementation of the basic conformance level of the XSL FO standard. So, if you convert RTF to XSL FO for further use of FOP 0.20.3, you must use the compatibility option
     -o fop-0.20.3
in the RTF TO XML call.