You can Zoom-in or Zoom-out in Notepad++ using,
1. Control + Mouse wheel Scroll-in or Scroll-out.
2. Keyboard Ctrl+Numpad Plus(+) to Zoom-in or Ctrl+Numpad Minus(-) to Zoom-out.
But many a times we Zoom happens by accident. Ctrl+Mouse get pressed so Zoom takes place instead of Scroll. In order to disable this functionality you need to download a plugin called "Zoom Disabler"
Installing Zoom Disabler
1. Goto Menu -> Plugins -> Plugin Manager
2. Look for Zoom Disabler and Select the Check-box and Install
3. After Installing the Plugin Notepad++ will restart and "Zoom Disabler" Will be available under Plugins Menu.
4. Try Ctrl+Mouse Well in or Out. Text will remain unaffected.
When you install this plugin, by default it Disables Mouse Zoom. If you want to enable it again you can do it by going to Menu -> Plugins -> Plugin Manager -> Zoom Disabler and de-select Disables Mouse Zoom