Fully agree with Richard. Since I am uncertain whether the general crowd talk of the same thing, let me take a step back.
The fact is, there are more than two things:
1. (printed and bound) books
These were very common in the analog world, so much so that the word 'book' stood for them. See https://en.wikipedia.org/wiki/Book
. Now in the digital (virtual) world we need to say analog books, I suppose.
2. Electronically prepared documents
2.a Optimized to be printed and bound
The universal format used to be PostScript, now PDF is the most common.
2.b Made for the computer screen
There are tons of formats almost always described in a mark-up or mark-down language https://en.wikipedia.org/wiki/Markup_language
- "HTML" in a wider sense. The Moodle activity Book also belongs to this category, together with Wiki, Blog
Also mark-up combined with other web technologies to get "paper-like" formatting, still able to adjust for the variing display sizes.
Now to the original question:
> At the moment in our courses/modules for content we use Moodle books, which work fine at the moment. It's nice to have content broken down into pages, with a menu on the left.
So you are happy with 2.b.
> However how would you compare this to PDFs? The menu with table of content can be displayed. Also more accessible maybe? Furthermore easier for students to download (instead of downloading books with funny formatting).
Obviously. The 2.a and 2.b are two different worlds. Web is not paper!
> Can you provide your thoughts on the following questions please?
> What do you use - books, pdfs or something else?
If you need both, you need to adapt new techniques. One of mine, now abandoned, is documented here https://moodle.org/mod/forum/discuss.php?d=276119
P.S. I am taken aback hearing that "quick to update" is a main criteria for the selection of technology. I never had to lead students towards moving targets!