Finicky pdf's in Preview

I use Preview regularly to combine or separate pages of documents so I can save them individually. Preview is perfect for this... however sometimes I get those pdfs that won't let me move pages and won't even let me save it after making any changes, such as deleting pages altogether... the save options are gray'd out. So....... WHY??? In case it's of importance it's usually pdfs that have been saved directly from a form generator website such as Zipforms. http://www.mac-forums.com/forums/mac_images/images/smilies/Not-Amused.png