Sorry, I'm too late.
As with the Java part of `pax`,
it may be possible to extract annotations from PDFs with a C/C++ program.
But, it looks like `latex-pax` is adding the extracted annotations
to the PDF using `pax.sty`.
The `pax.sty` is for LaTeX, and it looks like it is for pdfLaTeX only.
It would be quite difficult to get it to work with Texinfo on XeTeX/pdfTeX.
Alternatively, you could ask the xdvipdfmx developer
to add an option to keep the links and the annotations.
> Look at the documention of `\with-url` in the PDF version of the NR:
> The link in the included snippet (to 'lilypond.org') doesn't work.
> This is a limitation of pdfTeX (or XeTeX), which drops all links and
> annotations for included PDF files.
>
> Masamichi-san, is it possible to extend `extractpdfmark` to cover
> this, too? Looking around it seems that the tool 'pax'
> ('PDFAnnotExtractor') in TeXLive can do that. However, it is based on
> Java...
>
> https://github.com/bastien-roucaries/latex-pax
>
> The reason why I ask is the planned inclusion of the Visual Grob Index
> in the NR – all grobs shown have links to the corresponding
> documentation. If these links are lost the value of the index is
> reduced enormously, and it would be probably better to not include it
> at all but to have a stand-alone, simple, two-page document instead.
