We are going to suggest the package as alternative/replacement to kde-apps/okular[chm] so it should be available to the same arches.
On ppc64 it can only be keyworded for ppc64le as dev-qt/qtwebengine-5.15.2_p20211019 is masked on ppc64 (Big Endian).
arm64 done
ppc64 done, closing. thanks for direct CC