Database Software

I have an existing scheduled batch PL/SQL process that generates a flat-file interface to another system. This procedure must be updated to produce XML which will be transformed into either a PDF document or another flat file interface. I will be using XSL Formatting Objects to do the transformation. I am not sure how best to generate the XML. It must remain in PL/SQL. The XML document is quite complex and cannot be generated using one SQL statement. I am not sure if I can use XSU or must generate the XML tags and values using UTL_FILE. Any suggestions?
Will it be necessary to generate an XML file or can the transform be called within the procedure? I do not need to retain the XML.