I read the abstract but I'm still a bit confused. Will this help people who have down syndrome? Or is it a way to help future pregnancies?

The realistic reading is that: In the future, a treatment based on this technique could help parents going through IVF rescue more embryos from trisomy-21.

Considering that scope, the people for whom this could be useful are those who have very few embryos, one or more of which have trisomy-21. With a young couple, they will have many embryos and preimplantation genetic testing will reveal trisomy-21 early enough that even if they have fertility issues they can just run more rounds of IVF.

With an older couple or one with severe fertility issues, they may only have one or two embryos to work with.

This is all science-fiction, though, since a technique like this will require a lot of work (both in development and in regulation) before it can go live.

Future pregnancies.