From the Knowledgebase

In the Premium version there is nowhere to enter the license key

In Settings -> PDF Embedder within your WordPress site, there should be a License tab, and there you can enter the license key you received in your purchase email.

If there is no License tab, then you are almost certainly still using the free version of the plugin. In the Plugins page, Deactivate the free version and follow the instructions to install the Premium plugin you have purchased.