If you have a short-term project (e.g., grading exams, reviewing a contract), just use the free trial. Install it, do all your work, uninstall it. You can do this once per computer (or use a virtual machine to reset).
If you buy a new PC, you don't need to buy a new key. Simply uninstall PDF Annotator from your old machine and use your existing license key to activate the software on the new one. If you've lost your key, the official website offers a tool where you can enter your registered email address. Version Upgrades pdf annotator license key