I have 3 problems with -outline mode.
1) it has encoding problems and can generate error message such as:
error : xmlEncodeEntitiesReentrant : input not UTF-8
2) the produced XMl outline has @id that start with a number. This is not XML-compliant.
3) the coordinates are not along the same scale as the main XML produced by pdf2xml. So, one has to load both the outline and the doc XML to compute the correct x,y,w,h using the page width and height.