If you own an old iPhone, iPad that you do not use any more, you can turn it into a second screen for your PC using a screen sharing program, such as Deskreen. This is a free and open source software program which allows us to use any device with a web browser as a second screen for computer via Wi-Fi. You will not only be able to mirror the whole PC screen to your iPhone or iPad, but also selectively share certain program window on your computer. See how to mirror PC screen to iPhone iPad for free using Deskreen below.
Share computer screen with iPhone iPad wirelessly
First of all, go to download the screen sharing software Deskreen here. Then install it on your Windows computer. Run the program on your PC, you will see a QR code.

Connect your iPhone or iPad to the same Wi-Fi as your computer. Then use it to scan this QR code. Or you can manually type the web address in your web browser on the iOS device to access Deskreen for web. The connection request will be sent to Deskreen on your computer. You will then see the prompt in Deskreen on your PC with your device information such as IP address, OS details, browser information, and so on. Click Allow button to accept the connection. Deskreen will now ask you to select the screen sharing source, you can choose either to share the whole computer screen or certain program window on your computer.

Preview the window or screen to be shared in Deskreen on your computer and finally hit the Confirm button to start sharing your PC screen with your iPhone or iPad.
Sharing computer with Android, another PC or Mac
In fact, you can use Deskreen to share your computer with just any other smart device or computer which is able to run a web browser, such as Android smartphones, Android powered tablets, another Windows PC, Mac, smart TVs, etc.
Deskreen also comes with different versions for Mac and Linux. That’s to say, you can also use it to share your Mac and Linux computer screen with other computers or smart devices.