You can installed PDFTK using Robert's repo. Run following commands:

//copy robert-gcj-epel-7.repo to /etc/yum.repos.d folder
wget https://copr.fedorainfracloud.org/coprs/robert/gcj/repo/epel-7/robert-gcj-epel-7.repo -P /etc/yum.repos.d

//robert-pdftk-epel-7.repo to /etc/yum.repos.d folder
wget https://copr.fedorainfracloud.org/coprs/robert/pdftk/repo/epel-7/robert-pdftk-epel-7.repo -P /etc/yum.repos.d

yum install pdftk

Thanks