Pdftk is not public domain software.  It is licensed to the public under the GNU General Public License (GPL).  Pdftk uses third-party libraries which have their own licenses.  The exact licensing terms can be found in the third_party folder of the pdftk source code and on the pdftk license web page: http://www.pdflabs.com/docs/pdftk-license/.  Source code for pdftk and for these third-party libraries is available from this same page.

If you write software that uses any portion of pdftk and plan to distribute it along with your software, you will need a commercial license.  The exception to this rule is if your software is licensed to the public under the GPL or another compatible license.

Commercial licenses include commercial support and allow commercial redistribution. Please email sid.steward at pdflabs dot com to learn more. And please include pdftk in your email subject.