scan all the fronts, then the backs. stitch them together and ocrmypdf them.

scan.sh

#!/usr/bin/env nix-shell
#!nix-shell -I nixpkgs=/home/grahamc/projects/github.com/NixOS/nixpkgs
#!nix-shell -i bash -p sane-frontends netpbm imagemagick poppler_utils ocrmypdf


set -x

make_pdfs() (
scanadf \
  -d pixma:04A91774_21C8AC \
  --source "Automatic Document Feeder" \
  --mode color \
  --resolution 300 \
  -l auto \
  -t auto

  rm ./*.ps ./*.png ./*.pdf
  for f in $(find -type f -name 'image-*' | sort -n); do
      if ! pnmtopng "$f" > "$f.png" ; then
        rm "$f.png"
        continue
      fi

      convert "$f.png" "$f.pdf"
      rm "$f.png"
  done
)

(
    cd fronts
    make_pdfs
)

(
    cd backs
    make_pdfs
)

(
    mkdir unite
    cd unite

    i=0
    while true; do
        i=$(( i + 1 ))
        front=$(find ../fronts -type f -name '*.pdf' | sort -n | head -n1)
        if [ "x$front" != "x" ]; then
            mv "$front" "./page-$i.pdf"
        fi

        i=$(( i + 1 ))
        back=$(find ../backs -type f -name '*.pdf' | sort -nr | head -n1)
        if [ "x$back" != "x" ]; then
            mv "$back" "./page-$i.pdf"
        else
            break
        fi
    done
)

pdfunite $(find ./unite -type f -name '*.pdf' | sort -n) ./united.pdf
ocrmypdf \
    --clean \
    --rotate-pages \
    --deskew \
    --remove-background \
    --sidecar ./scan.txt \
    ./united.pdf \
    ./output.pdf

exit 0