is there a way to drag & drop pdfs on my Windows10 computer into the iCloudDrive directory
See -> Upload and download files from iCloud Drive on iCloud.com
Upload and download files from iCloud Drive on iCloud.com – Apple Support (UK)
Sure you have Apple Files app on your iPad -> https://apps.apple.com/us/app/files/id1232058109
Tap Apple Files app, you wil see the pdf files, but it is not possible to open them with Books, there is no option when you tap on a pdf file. You can't move either to Books.
Is there a way I can add a directory for the iBooks?
No, it is not possible, because iBooks is not available (more) for Windows OS computer.
I already turned on synchronisation via iCloud for Books on my tablet and they take up space in the iCloud, so I am certain, that it is the connection from the iCloud to my WindowsPC that is causing the issue here.
See -> Manage your iCloud storage
Manage your iCloud storage - Apple Support
To reduce your iCloud storage see also -> Connect external storage devices to iPad
Connect external storage devices to iPad – Apple Support (UK)
If the iPad has an Apple Lightnng connection port, you can use a SanDisk iXpand flash drive to use as an external hard drive. They are available in various sizes.
You can also use that iXpand flash drive as a tool to save the photos or pdf files to a real external hard drive afterwards.
Save your pdf files on iXpand Flash Drive.
iXpand Flash Drive are available at Apple Store and at Apple Authorized Service Providers.
iXpand flash drive app can be downloaded from the App Store.
See -> SanDisk iXpand-flashdrive Go
SanDisk 64 GB iXpand Flash Drive Go - Apple
Then download the SanDisk iXpand Drive app from the App Store, see ->
https://apps.apple.com/us/app/sandisk-ixpand-drive/id1079870681
