Can't combine PDFs in Preview 7.0
I just upgraded to Mavericks last night, and now I'm having an issue with Preview -- I can't use it to combine two PDF documents.
The help section says to open the documents you want to combine and set both to thumbnail view, then drag the thumbnail of one doc on top of the thumbnail for the other doc and release it when a green "+" symbol appears. This method worked fine in Mountain Lion, and I used it regularly. Now, the green "+" symbol doesn't appear, and I can't combine the documents.
Is anyone else having this problem? Does anyone know how to fix it? Or is it an error that Apple needs to patch?
MacBook Air (13-inch Mid 2012), OS X Mavericks (10.9)