Okular is a document viewer that supports file formats such as like PDF, Postscript, DjVu, CHM, XPS, ePub and others. Easy to use, it is the default document viewer in Kubuntu but can also be used on Windows, macOS, *BSD, etc.
Okular comes preinstalled in Kubuntu and can be found under Menu/Applications/Graphics or by searching in the menu.
If, for some reason, it is not installed it can be reinstalled by opening Konsole and typing,
sudo apt install okular