Right you are, so back to the original question about exporting an invoice as xml, there is no point using a standard report - unless you are prepared to use a plugin of some sort to convert the pdf to xml.
If you design a report and make sure it contains only text, and then EXPORT the report to Invoice1234.xml you should end up with a file containing the text
A report only becomes a pdf document when you PRINT or Export a *.pdf , but in this case you are exporting the document with a *.xml
I could be totally wrong of course, the exported file may contain other characters which totally debunks my theory. If I had time I would test it out, maybe you should give it a try (My experience using the report writer for this purpose has been for a simple text layout, no grid items and using Text document type)
