I\'ve been using PDFTK Server on OSX pre 10.11 for over a year without any issues running commands on the command line.
PDFTK
OSX pre 10.11
After installing OS
@Learner I fixed the recipe here: https://github.com/zph/homebrew-cervezas/blob/master/pdftk.rb and submitted a PR to the repo you linked.
Until that's merged, you can:
brew install https://github.com/zph/homebrew-cervezas/blob/master/pdftk.rb