Based on https://github.com/millie/heroku-buildpack-ruby-pdftk, but modified for use with https://github.com/ddollar/heroku-buildpack-multi.
How to install:
Download PDFTK source (compiled for Heroku's Cedar Stack) from http://github.com/millie/pdftk-source
Upload the tar.gz to your own S3 bucket. Make it public. Remember the S3 URL of the tar.gz file.
Clone this repo to your own.
Update lib/custom/pdftk.rb source_url method to return the S3 URL from step 2.
Add your cloned repo to your project's .buildpacks file.
Add config vars to heroku like so
heroku config:set \ BUILDPACK_URL=https://github.com/ddollar/heroku-buildpack-multi.git \ PATH=[your current PATH var]:/app/vendor/pdftk/bin \ LD_LIBRARY_PATH=[your current LD_LIBRARY_PATH var (if you have set before)]:/app/vendor/pdftk/lib
Copy the snippet above into CLI.