In FM Pro 11, when we embedded PDF files into container fields, we could open the PDF's by double clicking on the container field.
This was only the case if you used insert object or did a drag and drop to embed the file as an object--either way, FileMaker 11 in windows used OLE (Object Linking and Embedding) to open the file when it was double clicked.
OLE and Insert Object are not supported in FileMaker 12 so this behavior is not available to you in FileMaker 12.
Double clicking a container field opens the file in the container only if the file was inserted as a reference using Insert File. The other insert methods (except for Insert Object in Filemaker 11/windows) do not produce a result that you can use to open the file by double clicking the container field--neither in FileMaker 11 nor 12.