Lately I've been thinking about using my old nexus 7 (from 2012) as a second PC monitor.
I've read articles regarding android apps for this purpose as well as comments from Google Play. Some examples can be found here and here. Bottom line, you have to pay and most likely you will end up struggling with (huge?) lag and/or compatibility issues. I tried splashtop but it felt lacking in some way, it was just mirroring my main monitor and it was free just for the first 5 minutes.