According to KDE Documentation → Common Keyboard Shortcuts → Panning and Zooming there are 4 keyboard shortcuts for resizing/panning a window, where Meta+◀─ and Meta+─▶ also move the window to another monitor in a multi-monitor environment.
However, is there really no shortcut to maximize a window on the screen it's currently located on? Can such a shortcut be configured somehow?
I found that Meta+PgUp switches between maximized and windowed/restored size.
I filed an according bug and it has been resolved and fixed in the documentation source already.
Simply searching for "maximize" in shortcuts settings shows the shortcut you're looking for.
The default is Meta + Page Up.
The documentation page has moved.
Look here for Panning and Zooming: KDE Documentation → khelpcenter → KDE Fundamentals / Stable (Frameworks 5) → Common Keyboard Shortcuts
Or use your system: Open Application Launcher > System Settings > Shortcuts
Then type kwin to view all shortcuts or max for maximize window in the search bar. Here you can customize the shortcut maximize window / restore window Meta+PgUp to another if you like.
