I have two responses, one more general than the other, and neither may be what you are looking for.

Generally, when one has an object with multiple components, rather than multiple objects in a group, ungroup will not work but "Break Apart" in the "Path" menu (may first have to use "Object to Path" at the top of the same menu) sometimes does the trick.

More specific to your query, if you have access to the LaTeX the PDF was generated from, there are several ways of varying niceness to use it in the context of Inkscape: wiki.inkscape.org/wiki/index.php/LaTeX

Hope this at least leads you to something that helps.

-Arlo James Barnes