There is bad link on the docs page for Krita. Unfortunately, there’s no simple way to tell them they screwed up their page.
==============================
You might need to add the “download” attribute on the download link for your epub doc (link on this page: docs.krita[DOT]org/en/) or have the sys admin adjust the the mime type for epub on the server. It should be application/epub+zip. It’s probably using the default text.
Chrome 98.0.4758.102, Opera 82.0.4227.58, and Edge 98.0.1108.56 all render that link as a text file in the browser window rather than downloading it.
I tried on my Android phone with 3 different browsers (Long press, then “Save Link”… I’ve tried using Dolphin, Samsung Internet, and Opera) and then tried using Google Play Books to open them. Each time, it said that the epub was either corrupted or incompatible.
On my computer, I do right-click on the link then Save Link As. That gives me an 82MB download of KritaManual.epub in my Downloads folder.
My e-book reader displays it with no problem.