Related - this might be helpful to someone.

ImageMagick can do a visual PDF compare:

    magick compare -density "$DENSITY" -background white "$1[0]" "$2[0]" "$TMP"

(density = 100, $1 and $2 are the filenames to compare, $TMP the output file)

You need to do some work to support multiple pages, so I use this script:

https://gist.github.com/mbafford/7e6f3bef20fc220f68e467589bb...

This also uses `imgcat` to show the difference directly in the terminal.

You can also use ImageMagick get a perceptual hash difference using something like:

    convert -metric phash "$1" null: "$2" -compose Difference -layers composite -format '%[fx:mean]\n' info:

I use the fact you can configure git to use custom diff tools and take advantage of this with the following in my .gitconfig:

    [diff "pdf"]
        command = ~/bin/git-diff-pdf

And in my .gitattributes I enable the above with:

    *.pdf binary diff=pdf

~/bin/git-diff-pdf does a diff of the output of `pdftotext -layout` (from poppler) and also runs pdf-compare-phash.

To use this custom diff with `git show`, you need to add an extra argument (`git show --ext-diff`), but it uses it automatically if running `git diff`.