Fatal error: Class 'TCPDF' not found
When you manually install the extension instead of following the recommended installation via Composer, not all dependencies are installed. In this case not using Composer meant the TCPDF component is missing.
To address this issue please delete the app/code/Fooman folders that you just created and then go through our installation guide as outlined here: Installing Pdf Customiser (M2) - from Fooman website
Using Composer ensures that all dependencies are installed and autoloaded correctly.