I’m not sure about android/ios (I looked at their faq and it sounded like it might be different), but with Windows at least, I don’t think you can download them (in the sense of like, getting a file you can move around). You get a library, and you can access the books in your library through their browser app only. It’s a lot like Steam. I don’t know of a way (on Windows) to ‘preload’ if worried about download times or to read offline.
Free stuff you can just click a couple buttons and it’s added to your library, which is a lot of how mine has ballooned… I’m not very picky.
One thing to watch out for though is that most of the free stuff has the label 【期間限定 無料お試し版】- I believe these are a separate version of the book that disappears from your library after a time limit expires.
I try to avoid these because they clutter up the library and I’m never going to get around to reading them in time anyway, but they could be useful if you want to try something out on the spur of the moment.
Ones like this (picking a random one from the front ‘free’ page), on the other hand, are free to keep forever (or you know, until the platform collapses or they revoke your license to it anyway):

There’s no 期間限定 label, and the price is a temporary promotion, not the listing itself.
There’s definitely fewer of this latter type, but still a surprisingly large number.
It can be kind of a pain to sort through, but it’s definitely possible to enjoy trawling through and accumulating free stuff! There’s a thread somewhere around here where someone was posting free diamonds in the rough… I might post there from time to time when stuff stands out (right now, sadly, I would not describe it as a sales bonanza)
And they have sample pages, so you can quickly check if the level is to high or not.
