Obviously without any proprietary service.
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.
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
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.
A loosely moderated place to ask open ended questions
If your post is
it’s welcome here!