It's browsable, but there is no search and the in-browser search does not work because the pages are only rendered when scrolling. So, like many current web pages, pretty and useless.
Not to mention that on my machine, it took about 30 seconds to load and draw anything except empty pages. Just marvellous.
I think you are not supposed to load the Z80 cpu manual PDF on an actual Z80 : )
It renders on my Pixel 3 in under half a sec. Seems ok to me.
For me it is unusable because default zoom level is completely unreadable and there is no obvious UI to change it (although somewhat surprisingly ctrl-scroll works).
To zoom, ctrl+ works for me in the desktop browser, as for the web in general.
But, as miahi says, it's not really searchable. The links work fine though. I didn't notice any load time issues.
Also, with JS allowed there is a scrollable TOC on the left side, from where you can jump to the page.