Zypper hides in /usr/bin. If pdftk is in one of the configured repositories:
zypper in pdftk
will install the package. If it isn't, then you have a couple of options.
Option 1: Skip Zypper entirely
Get an RPM of the package, and install it with rpm, not zypper.
Option 2: Create a YUM repository
The more robust method, as it would allow the same package to be installed on all servers. Create the repository, add it to each of the servers, and then zypper's auto-update process will do the rest. Automatically (presuming you have auto-update turned on).
The hard part is picking your update server. Once you've identified which server will function in this role, figure out if you want to serve the updates over HTTP, NFS, or FTP.
Creating the repo. A good guide can be found here:
Ignore the bits about "smt-setup-custom-repo", you're just interested in the 'createrepo' parts.
Once you have a directory, drop your RPMs in there. Share it how you need to (I like NFS, it's simple and fast). Then on each server run the following
zypper sa updateserver:/path/to/repo "Name of Repo"
That'll add the repository to Zypper. Then you can just issue
zypper in pdktk
on each to install. If you have auto-updates turned on, you can auto-update your packages just by updating them in the repository. They'll apply to your servers as they auto-update.