SlackBuildsOrg/office/pdftk

If PDF is electronic paper, then pdftk is an electronic
staple-remover, hole-punch, binder, secret-decoder-ring, and
X-Ray-glasses. Pdftk is a command-line tool for doing everyday things
with PDF documents.  Keep one in the top drawer of your desktop and
use it to merge PDF documents, split PDF documents, apply watermarks,
repair corrupt PDF, update PDF metadata, and more.  pdftk does not
require Acrobat and is free software.

This SlackBuild repackages the newer Java port of the original pdftk.
A Java Runtime Environment of version >= 8 is required
(ex. zulu-openjdk8).