Put the .PDF (or .epub, if you’re feeling fancy) on your website. Indicate in it that the work is governed under something like a Creative Commons license. Probably the easiest way, at least if you have your own website.
Just add a licence.
I’ve published one with LaTeX on Gitlab, and stuck in a GPL licence since it’s the source files that I want to make free.
If you just want it to be shareable as a pdf, then CC-4 or CC-0 licences work better.
I’m assuming you mean to exclude even free proprietary. The reason I ask is there are, for example, many Comp Sci books published with open licenses on things such as GitHub.
You might be able to find some places to host by looking at https://creativecommons.org/, I haven’t dug through the site completely. There is a “share your work” section.
only partially relevant to your question but I thought this might be of interest:
“Easy to use, Open & Decentralized Content Distribution”
version 1 spec is based on atom and draft version 2 is based on JSON-LD
A loosely moderated place to ask open ended questions
If your post is
it’s welcome here!