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
To change the resolution from a remote desktop, use the Horizon Client Tools to display the Options dialog box and tap the Display Resolution setting. In Horizon Client 4.0 and later, you can also change the resolution from the Horizon Client Settings screen. Tap Settings at the bottom of the Horizon Client screen and tap Resolution.
Certain options, including 3/4 Scaling and No Scaling, are not available on iPhone 6 when the device is in zoomed mode. To display these options, you must exit zoomed mode.
Screen Resolutions for Using Projectors
Use the Display Resolution setting (Options dialog box in a remote desktop) or the Resolution setting (Settings screen in Horizon Client 4.0 and later) to set a larger resolution for projectors.
To display the keyboard and an expanded onscreen touchpad on the device while displaying the remote desktop on the projector or attached monitor, enable the Presentation Mode setting in the Options dialog box in the remote desktop. 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.
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.
Use the Keep the screen alive during Presentation setting to keep the display from turning off after a period of inactivity while in presentation mode. You can enable this setting from the Options dialog box in a remote desktop. In Horizon Client 4.0 and later, you can also enable this setting on the Horizon Client Settings screen. To display the Settings screen, tap Settings at the bottom of the Horizon Client screen.