The LXD documentation is written in markdown
LXD has been configured so that it automatically uploads to ReadTheDocs (LXD)a fresh set of generated HTML documentation (markdown→HTML) upon a new commit.
ReadTheDocs (LXD) has a section to download the HTML files at https://readthedocs.org/project/lxd/downloads/ but those tarballs with HTML files have not been updated since 2016. It appears there is some issue there because those HTML files are successfully generated and uploaded to the online HTML website. There is just an issue with compressing them into a ZIP file and uploading them to https://readthedocs.org/project/lxd/downloads/
@stgraber mentioned that there has been a technical issue with generating PDFs, so PDFs are not available.
Having said that, it is always possible to setup a container to generate offline documentation for you, whether that is HTML files (see the commands to generate the documentation) or even man pages.
Also, there are several projects on github that can convert from markdown to PDF.