Centos 7 has dropped it support for the libgcj making it impossible to install pdftk. Fortunately, there is a hack. Add a following repository and install pdftk as shown below:

#Add a repository
→ sudo wget https://copr.fedorainfracloud.org/coprs/robert/gcj/repo/epel-7/robert-gcj-epel-7.repo -P /etc/yum.repos.d
→ sudo wget https://copr.fedorainfracloud.org/coprs/robert/pdftk/repo/epel-7/robert-pdftk-epel-7.repo -P /etc/yum.repos.d

#Install 
→ sudo yum install pdftk -y