man
The man KIO worker lets you read the man pages installed on your system. It's easy to use:
man:: See the sections of the manual, and click to find the rest.man:/fopen: See the man page offopen.man:/fopen(3): See the man page offopenin section 3.man:/(3): See the index of section 3.man:/intro(3): See the introduction of section 3.
[!NOTE] If more than one man page has the name you entered, you get a list where you can choose the one you want.
There's also a shortcut: #fopen has the same effect as above.
If you don't find all your man pages, adjust the configuration file /etc/manpath.config, or a file of a similar name depending on your distribution, or adjust the environment variables MANPATH and MANSECT.
As with any other KIO worker, you can enter a URL like man:/socket in applications that open URLs, and you'll see the man page in HTML format.