Can you remove PDF's after you add them to iBooks? I have iCould sync on. So all my books get sync across my devices. Great!

Now, my question is: can I remove the PDF's (from my Downloads folder) after I added them?  

I tried it, it seems to work, but im not sure. Unlike with music you can right click on the book (in iBooks) and click Show in Finder.
All the books that you move to iBooks stored in

~/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks/


iBooks make a copy of every books that you move on it.

With iCloud Sync the Documents are stored in the following location - Thanks to @Rogier

~/Library/Mobile Documents/iCloud~com~apple~iBooks/Documents/


Personally I recommend that you make a backup or move your original ebooks on an external drive.