As far as I understand this the "standard" is the XML that is being embedded inside PDF, so you need a small tool to extract the XML from the PDF and then you'v

As far as I understand this the "standard" is the XML that is being embedded inside PDF, so you need a small tool to extract the XML from the PDF and then you've got the standard XML invoice. The PDF is just for nice presentation. I did a similar approach of using XML Stylesheets (XSL) to render the standard invoice as HTML when opened, this also looked nice.