The package is for extracting a TOC when the document does not yet contain one.
But it includes a command also to extract the TOC if the document already has one
(so that it can be altered)
I am working on some alternative/upgrade to doc-view, pdf-tools and djvu.el, although
it might take a while as it is a lot of work. I did not make a project page yet, but will
probably post a link to it soon here. Indeed, for now it provides imenu commands for
pdf and djvu documents. If you like, I could drop the code for creating the imenu index
here (you have it already of course, but you might like to see different solutions). Just,
let me know if you are interested.