Download Pixelmator Clasic for FREE from here.
Install by dragging and dropping in the Applications folder
Click Try and use all the features of Pixelmator for free.
When you are nearing the end of the Trial like the image below
Open terminal and run this command
sudo rm /Users/username/Library/Containers/com.pixelmatorteam.pixelmator.trial/Data/Library/Preferences/com.pixelmatorteam.pixelmator.trial.plist
Now reopen the Pixelmator App.
Trial is now reset to 30 days again.
Disclaimer: This tutorial is for educational purposes only. We condem piracy and in no way should be used to pirate the software