Fortunately there is pandoc-bin at the AUR.
For anyone else interested, the same solution applies to shellcheck - just install shellcheck-bin from AUR.
More importantly (for any haskell developers using Arch), there are also static builds for stack, cabal and ghcup too although you then need to maintain your own set of GHC and libraries.
I can't imagine what it must be like to use the official packages since the haskell library ecosystem is quite fast moving.