Two monitors are better than one, and with macOS Catalina you can turn your iPad into a second screen with just a few clicks thanks to a new feature known as Sidecar. But Sidecar is limited to certain Macs and iPads, which is a shame if you have an older device (or a Windows PC). Fortunately, a few third-party apps can fill the void on other PCs, Macs, iPads and Android tablets.
The ideal option: sidecar on macOS Catalina
Sidecar is perhaps the best option to turn your iPad into a second screen. So if you have a Mac and an iPad for the past three or four years, you want to try it first. (If your devices are older, continue to the next section.)
To connect your iPad as a display, connect your iPad to your Mac via USB, or ̵
Then click on the AirPlay button in the menu bar of your Mac and select your iPad from the list. The icon changes and when you click on the menu again, you get a number of options. You can mirror or extend the screen of your Mac, show or hide the sidebar and the Touch Bar on the iPad, or disconnect your tablet.
You can use Sidecar wired or wireless, but if you want to switch between them, you must disconnect and reconnect via the AirPlay menu . Apple says it should work wirelessly as long as the iPad is within 10 meters (30 feet) of the Mac, and you can even use your Apple Pencil to sign documents and draw in supported apps. View the Sidecar manual from Apple for more information.
The cheap cross-platform option: Splashtop Wired XDisplay
If your Mac or iPad is not supported by Sidecar – or You are a Windows and Android user – Splashtop Wired XDisplay is a third-party app that can turn your tablet into a second monitor. It only works when it's connected via USB and isn't the smoothest option I've used, but it won't do the job for a lot of money.
Splashtop Wired XDisplay has two components: a desktop agent that runs in the background on your PC or Mac and an iPad app or Android app that runs on your tablet. (The iPad app appears to be completely free, while the Android app costs $ 7 – although there is a free trial that works 10 minutes at a time.)
After installing both the desktop and tablet apps, check If the desktop app is running on your computer, connect your tablet to your computer via a USB cable and then launch the XDisplay app on your tablet. A connection to your computer is established and a blank desktop appears on your tablet. You rearrange your screens via the screen settings of your computer, drag windows to your tablet and otherwise use it as a second screen for a productivity increase.
I recommend having a stand to keep it upright – something works great if you use your tablet with a laptop, while a more adjustable stand like this is ideal for desktop use.
Splashtop Wired XDisplay works reasonably well, at least in my experience with an iPad and a Windows PC. There is a noticeable latency and I had some problems with the mirrored mode crashing on one of my computers, but for the price of free it is worth a try. (The $ 7 Android app has more mixed reviews, and if you're going to pay money, I think it's worth shooting a few more and using our paid recommendation, Duet Display, below.)  A more expensive but better alternative: Duet Display
Splashtop Wired XDisplay is reasonable for a free option, but if you want the smoothest possible experience, Duet Display is the best app I have tested to turn an iPad into a second monitor.
Like Splashtop, it has an app for your tablet that connects to an associated app on your computer. The desktop app from Duet Display is free, but the iOS app costs $ 10 and the Android app costs $ 20 (because it is brand new at the time of writing). That version of the app only works when connected via USB, but adds a Touch Bar to every Mac, supports iPad keyboards, and adds touch control to your Windows or Mac apps on the tablet screen.
If you want more functions, there is also an in-app "Duet Air" subscription that adds wireless connectivity, remote desktop and enhanced graphics for $ 20 a year, and a "Duet Pro" subscription that adds drawing functions for $ 30 a year.
Once you installed both the desktop and tablet apps, make sure that the Duet Display desktop app is running on your PC in the background, then connect your tablet via USB and launch the mobile Duet Display app. It should immediately connect to your computer and you will see a desktop appear, ready for use.
Finally, click the Duet Display icon on your PC to access a few other settings, such as the frame rate and resolution of your tablet. For me, the default settings worked fine, but if you find it a bit slow on an older laptop, it may help to lower the resolution, frame rate, or power consumption from this window.
If you have the Duet Air subscription, you can connect Duet Display wirelessly by opening the desktop app, going to Advanced settings and pressing the Wireless tab (in Windows) or the Air tab (in macOS). Your tablet should appear in the list and you can click it to connect. (If it does not appear, you can connect to it manually by typing in the IP address of your tablet.)
Duet display may be more expensive than its competitors, but in my experience it is by far the smoothest experience. If you regularly use your iPad as a second monitor, this is certainly worth the price.