Used to be in the repos, but dropped for some reason. If you are on F7 or before yum install pdftk is sufficient. Otherwise get the source rpm
then (as root):
rpmbuild --rebuild pdftk-1.41-5.fc7.src.rpm
There may be dependecies you need to install, particularly libgcj-devel, the rpm build output should tell you. Its an F7 package, but worked fine for me on F8
pdfedit (in the repos so yum install pdfedit) can also do it, but it has a really clunky interface and pdftk is IMHO much quicker for simple jobs.