When using Ubuntu, I cannot read CHM and EPub documents, even if I have okular-extra-backends and libchm1 installed. Why?
Ubuntu (thus Kubuntu as well) packages of Okular are compiled without support for these two formats. The reason is explained in this Launchpad report.
Why are the speak options in the Tools menu grayed out?
Because you don’t have a speech service on your system. Install the Qt Speech library and they should be enabled
有些字符没有被渲染。如果开启调试选项，那有些行会提到“缺少 xxx 的语言包”
安装 poppler-data 软件包