PangoPDF was initially developed as a component of the Sun xmlroff XSL Formatter (http://www.sourceforge.net/projects/xmlroff/).
The initial public release of PangoPDF was version 1.2.0, corresponding to Pango 1.2.0.
The initial PDF backend uses PDFlib because Pango and gnome-print were not always as aligned as they are today. At that time that the xmlroff project started, mapping Pango glyphs to gnome-print glyphs was not exactly straightforward. Choosing PDFlib, therefore, got PDF generation working quickly and allowed the rest of the xmlroff project to proceed.
The long-term preferred solution, however, would be to migrate PangoPDF to be a gnome-print backend for Pango.