Install the package calligra through pamac (Add/Remove Software) or
sudo pacman -S calligra
Installing it as a dependency on Okular’s pamac page will not work (it results in a "target not found: " error.) You must install it as a package.
The main reason I open .docx files in Okular is so that I can invert the colours and view a “dark mode” version of the document without making any changes to the doc itself.
Unsure as to if this is the right place for this but I couldn’t find any results on how to do this and had to figure it out. Besides the fact that these dependencies cannot be found (tested on two up-to-date machines) this is a very unintuitive process. For some reason there’s isn’t anything on this in the Okular handbook.
Please change the category if needed, I didn’t put it in Support as I already have the solution 