The XSL Transformation language lets you specify how an XML document should be converted into other XML documents or text files. One kind of XML document that is generated with an XSLT document is an FO document, which can then be further processed to generate PDF output. XMLSpy contains built-in XSLT processors (for XSLT 1.0, XSLT 2.0, and XSLT 3.0) and can link to an FO processor on your system to transform XML files and generate various kinds of outputs. The location of the FO processor must be specified in the XSL section of the Options dialog (Tools | Options) in order to be able to use it directly from within the XMLSpy interface.
XMLSpy also has a built-in XQuery engine, which can be used to execute XQuery documents (with or without reference to an XML document).
Commands to deal with all the above transformations are accessible in the XSL/XQuery menu. In addition, this menu also contains commands to work with the Altova XSLT/XQuery Debugger.