Is it possible to download the docs of a library as PDF? For instance, if I want to convert this full doc to a PDF, how can I do it.

You could download the repo from https://github.com/quantumblacklabs/causalnex/tree/develop/docs and use a mark down to PDF converter of sorts?

you can create a PR in the original repo to enable downloadable format. readthedocs supports epib/mobi/pdf/whatnot https://docs.readthedocs.io/en/stable/downloadable-documentation.html#downloadable-documentation