You can download the installer from https://www.pdflabs.com/tools/pdftk-server/
After installation, open a new terminal and check installation:
$ pdftk --version
pdftk 2.02 a Handy Tool for Manipulating PDF Documents
Copyright (c) 2003-13 Steward and Lee, LLC - Please Visit: www.pdftk.com
This is free software; see the source code for copying conditions. There is
NO warranty, not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$
