iBook bookmarks in PDF files

I have several PDF files that for an unknown reason will not accept bookmarks. The bookmark option is greyed out. Some of these are files I downloaded and other are ones I created in Acrobat Pro. Is there some setting in Acrobat that needs to be set for bookmarks to work in iBooks on the iPad?

Most of my PDF's accept bookmarks just fine.

Puzzeled in Texas.

Dave