You can use Horizon Client with external displays and you can change screen resolutions.
When you connect your device to an external display or projector, Horizon Client supports certain maximum display resolutions. You can change the screen resolution used on your device to allow scrolling a larger screen resolution.
Enlarging the Screen Resolution for a Remote Desktop
By default, the display resolution is set so that the entire Windows desktop fits inside your device, and the desktop icons and task bar icons are a certain size. If you change the default to a higher resolution, the desktop still fits inside the device, but the desktop and taskbar icons become smaller.
You can pinch your fingers apart to zoom in and make the desktop larger than the device screen. You can then tap and drag to access the edges of the desktop.
Changing the Display Resolution Setting
You can use the Resolution setting to set the display resolution to a larger value. If you are using a remote desktop or application in full-screen mode, tap the Horizon Client Tools radial menu icon, tap the Settings (gear) icon, tap General settings, and drag the slider in the Touchpad sensitivity option. If you are not using full-screen mode, Settings is in the menu in the upper-right corner of the Horizon Client toolbar.
Screen Resolutions for Using Projectors
You can use the Resolution setting to set a larger resolution for projectors.
You can use the Presentation mode setting to display the keyboard and an expanded onscreen touchpad on the device while displaying the remote desktop on the projector or attached monitor. The expanded touchpad and keyboard appear when you plug the device into the external monitor. The device detects the maximum resolution provided by the external display. Currently, the presentation mode feature is supported only if you have Android 4.2 on the following devices: Google Nexus 4, Google Nexus 10, and Samsung Galaxy S 4.
You can mirror the entire device display on a projector or attached monitor, including the Unity Touch sidebar, by turning off the Presentation mode setting. If you are connected to a remote desktop and the Presentation Mode setting is enabled, you can click Done to switch to mirror mode.
You can use the Stay awake setting to keep the display from turning off after a period of inactivity while in presentation mode.
To configure these settings if you are using a remote desktop or application in full-screen mode, tap the Horizon Client Tools radial menu icon, tap the Settings (gear) icon, tap General settings, and drag the slider in the Touchpad sensitivity option. If you are not using full-screen mode, Settings is in the menu in the upper-right corner of the Horizon Client toolbar.
Do not use an external mouse, such as a Bluetooth mouse, when using the Presentation mode setting. Instead, use the large onscreen touchpad on the Android device. If you attempt to use a mouse, the mouse pointer might not be able to move to the bottom or right side of the screen, and when moved to the top of the screen, might conflict with some of the function keys shown on the large onscreen touchpad, rather than the top of the remote desktop.