Even better, if your operating system has full-text file indexing, it'll be able to search them normally.
Edit: incidentally, a common way for this to work is that the documentation just comes with the software itself, so it's just sitting there on you computer. If the software updates, the documentation updates.
Websites like https://devdocs.io/ do this exact thing through desktop Progressive Web Apps.
Basically you keep your markup & source as static as possible, and allow a serviceWorker to cache those files locally. This can also be done with a SPA, but if we're talking docs, you get the benefit of only having to update a specific page if its raw html.
If you are a javascript developer chasing the bleeding edge of web dev, it's pretty close to real time!
Run this:
find /usr/share/doc -name '*.htm*'
You'll be surprised.Your system's package manager is installing the HTML documentation right along with the software you install.
When you update your system, new package versions along with their corresponding documentation are downlaoded.