The upcoming Inkscape 1.2 release supports multi-page PDF editing (import & export). Right now, it’s in beta, but was hinted at being scheduled for release this week. Inkscape 1.2 has a ton of other features too, but multi-page is one of the big ones, along with on-page autoalignment & snapping.
It’s on the Flathub beta remote, which you can add with:
flatpak remote-add --if-not-exists flathub-beta https://flathub.org/beta-repo/flathub-beta.flatpakrepo
(And then install Inkscape with flatpak install inkscape
and choose the beta version from flathub-beta
.)
…or you can just wait a few days. 
For PDF in general, there are a lot of apps that use PDF in some form on Flathub. I’m partial to PDF Arranger for manipulating PDF pages (combining, splitting, rearranging, etc.).
Update
Inkscape 1.2 has been officially released! The blog post has a great overview (nicer and more polished than the release notes on the wiki): What you’ll find inside Inkscape version 1.2 | Inkscape
The release video showcases so many of the huge new features in motion:
(This release is so huge, with so many things I’ve been wanting in Inkscape for decades. It should’ve been called Inkscape 2.0, IMO. Regardless of the version, it’s amazing!)
Also, there’s a pull request to bump up Inkscape to version 1.2 on Flathub, so the update will most likely land today on Flathub (and this will probably be faster than most distros).