• On https://github.com/jgm/pandoc/releases, you'll see a list of Pandoc releases.
  • Click one of them to see the available files in that release.
  • You can right click a given link to copy its URL to put that into a script. 
  • The gist of the following script (which you can put in your postInstall is to cURL  a modern pandoc release, move the binaries to where they'll be found, and deletes everything else:
#!/usr/bin/env bash
set -e

curl -L  https://github.com/jgm/pandoc/releases/download/2.8.0.1/pandoc-2.8.0.1-linux-amd64.tar.gz | tar xz
mv ./pandoc-2.8.0.1/bin/pandoc /usr/bin
mv ./pandoc-2.8.0.1/bin/pandoc-citeproc /usr/bin
rm -rf ./pandoc-2.8.0.1/
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