Installing pdftk on macOS
For whatever reason,
pdftk (installed using pdftk_server-2.02-mac_osx-10.6-setup.pkg)
seems to hang on recent versions of macOS.
The solution was to use this installer instead: pdftk_server-2.02-mac_osx-10.11-setup.pkg
This was linked from a Stack Overflow answer from Sid Steward of PDF Labs.
Quote: "I will update our PDF Labs site soon to reflect this update".
Considering that the above quote is from four years ago, I'm not holding my breath