Locating the iBooks folder in iCloud Drive (on Windows)
Summing up the answer in advance: the folder simply might not exist on Windows.
And the workaround in advance:
If I cannot locate the folder, the only solution is via extraction from the iTunes backup, see:
https://stackoverflow.com/questions/62434692/iphone-move-in-one-go-all-pdfs-generated-from-safari-page-prints-which-i-fr/62701384#62701384

I found a description that explains the equivalent Windows and Mac folders at https://deciphertools.com/blog/how-to-backup-ibooks-pdfs/.

Where are My iBooks PDFs Stored on my Computer?


On Windows, the iBooks PDF files copied to your iTunes by doing
Transfer Purchases are stored in your computer user's account folder,
alongside your iTunes music. Here's how to find your iBooks PDFs on
your Windows computer: Go into your user account's Music folder. (On
Windows 10, this would show up under This PC -&gt; Music.) Open the
folders iTunes -&gt; iTunes Media -&gt; Books. You should see your PDFs in
this folder.


On Mac, the iBooks PDFs copied by doing Transfer Purchases are tucked
away in data for the iBooks Mac App. Here's how to find your iBooks
PDFs on your Mac: Open Finder, and select the &quot;Go&quot; menu at the top of
the screen. Hold the Option key (or Shift + Option on latest Sierra
builds), and select the &quot;Library&quot; menu option from the Go menu. (On
OSX Lion and above, Library will only appear with the Option key
held.) From Library, navigate to Containers -&gt;
com.apple.BKAgentService -&gt; Data -&gt; Documents -&gt; iBooks -&gt; Books. You
should see your PDFs in this folder.

Then there comes an explanation of how to use iBooks for iCloud, but only on Mac:

First, we need to make sure you've got iBooks turned on for your
iCloud drive documents on your Mac.

From the Apple menu at the top of the screen, select System Preferences.
Select iCloud in System Preferences.
Next to iCloud Drive, press the &quot;Options...&quot; button.
Ensure the iBooks checkbox is checked.


The checkbox &quot;iBooks&quot; does not exist in my iCloud &gt; iCloud Drive settings. This could mean that it is simply not possible to sync iBooks with iCloud on Windows, as @MatthewBarclay commented above. This is also plausible, because my iBooks are backed up, and Apple does only one thing of the both, either sync or backup. And then, to answer the question, there simply is no such folder.
I have also tried opening it in terminal, as it could be hidden like the answer stated at Locating the iBooks folder in iCloud Drive, and I could not find it:
C:\Users\USER\Music\iTunes\iTunes Media&gt;dir /a
does not have it, showing both hidden and unhidden items with /a (for /a see https://www.computerhope.com/issues/ch001039.htm).
Perhaps my sync settings are corrupt or wrong. Because I marked iBooks to be included in iCloud, but they are in my iTunes backup instead (proven), and they can only be in the backup OR in the iCloud (not in both, thus, they seem to be ONLY in the backup), if I am not mistaken. Settings &gt; scroll down to Books &gt; iCloud Drive is switched on, that cannot be the issue, following https://www.topmobiletech.com/how-to-sync-ibooks-to-all-your-ios-devices/.
I will not test this anymore, please test yourself and set a different answer which I will accept in case you find more.
If I cannot locate the folder, the only solution is via extraction from the iTunes backup, see:
https://stackoverflow.com/questions/62434692/iphone-move-in-one-go-all-pdfs-generated-from-safari-page-prints-which-i-fr/62701384#62701384