This is really neat! For work, I've found myself from time to time exploring the tech around PDFs. I find this tech strangely fascinating. It's like a shim on top of something old and ugly that enables integration with much more modern systems.

Some quick feedback (and a shameless plug):

The CLI interface should output JSON. It would be nice to combin with a CLI JSON parser such as jq[0].

Shameless plug: I've been working on a PDF CLI aimed at making it easier to programmatically fill out PDF forms: https://github.com/adelevie/pdfq. It provides an interface and some wrappers on top of the main pdf form-filling tool, pdftk. For example, you can get json out of a pdf form like this:

    pdftk hello.pdf dump_data_fields | pdfq

Or you can generate FDF from a json file:

    cat hello.json | pdfq json_to_fdf

You can also fill a pdf without touching an fdf code:

    pdfq set foo bar input.pdf output.pdf

[0] https://stedolan.github.io/jq/