I ran across the following issue myself, after saving some PDFs to the Pages folder of iCloud:

In an iCloud folder (KEYNOTE), some files (PDF, Keynote) just won’t allow me to move them into any other folder. I try to drag and drop, but once I bring the file into another folder, I see the “circle+crossed line” icon that tells me I’m doing the forbidden. I CAN move some files (also PDF, Keynote), and some I just can’t.

It also doesn’t seem to matter whether they’ve been downloaded to my computer or not.

Sharing & permissions are set the same for files I can move and files I can’t.

https://discussions.apple.com/thread/251324258

After some testing, I found this issue only happens in the Pages folder of iCloud, when I created a folder in the Documents folder of iCloud, I was able to move these PDF files there. (Also when looking at the files via the open function in Pages program itself these non-movable files are ghosted out, indicated that Pages can’t open them.)