iBooks local files deleted after disabling iCloud

It would seem that iBooks functions very strangely when it regards PDFs downloaded to the device, depending on whether or not iCloud is enabled for the app.

I have downloaded to my iPhone 6s+ a few dozen PDFs which are crucial to my everyday work. However, iOS seems to enjoy deleting them -- to make space available to pre-download updates I really don't need -- I suppose based on the fact they are in iCloud, and therefore potentially accessible whenever I can get a reliable Internet connection. Which is not as often as Apple imagines.

So I disable iCloud based on the belief that Apple will no longer manage my disk space on my personal device. But, when disabling iCloud, apparently the PDFs which I took the precaution of downloading prior, do not only disappear from my iBooks shelf, but also are apparently deleted. Indeed, upon re-enabling iCloud for iBooks, all my PDFs are no longer downloaded.

Yet, when iCloud is enabled, selecting "Hide iCloud Books" in iBooks app itself, does not hide the entire library which is summarily deleted, as though they act as though they are iCloud-exclusives and thus not valuable to a non-iClouded iPhone.

To put it briefly, disabling iCloud is equivalent to wiping my iPhone clean of PDFs -- the reason for which is beyond me.

I therefore want to download my full PDF library from iCloud and load it into a 3rd party app that won't decide how to manage my personal files.

But I can't seem to find any iBook PDFs in the iCloud Drive app. iBooks does not offer any kind of transfer method to send PDFs.

The only thing that seemed promising was in 3rd party file managers on Mac OS, you can catch PDFs in the Books directory of the iPhone, but only for those few moments before iCloud "nabs them" into cyberspace, at which point I imagine the PDF is sent somewhere for which root access is required.

To cut it short, in what is becoming a long plaidoirie, does