Thank you for uploading the PDF, although I managed to download it using the curl invocation you provided, so at least their storage server works (website still errors 504 for me).
I also found out through this forum post, which I’d post to if it wasn’t closed, that here’s the User Guide’s source repo:
If anyone else was searching for it, one can find the PDF of the most recent 2020 edition under asciidoc folder there: