It's a shame that PDFtk is basically going away because the GCJ runtime has been deprecated by most major distros. PDFTK still lives on as a Snap. Also, there is a fork that uses openJDK instead. You can reference this Stack Overflow post:

https://askubuntu.com/questions/1028522/how-can-i-install-pdftk-in-ubuntu-18-04-bionic