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 are distributed under their own licenses: libgcj which is licensed under the GPL with an additional special exception, libgcc and libstdc++ which are licensed under the LGPL, GNU Classpath which is licensed under the GPL with the "Classpath Exception," and the iText library (itext-paulo) which is licensed under the LGPL.  The exact licensing terms can be found in the license_gpl_pdftk folder 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.  Note that these licensing terms cover both the source code and the binaries.

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 product support and are reasonably priced. Please email sid.steward@pdflabs.com to learn more. And please include pdftk in your email subject.
