I had the same problem. Here's how to fix it:
- Enter layout mode.
- Select your container field
- Go to the inspector, click on the data tab and scroll it all the way to the bottom to expose data formatting.
- Use the controls there to make your container field an interactive container field
- Return to browse mode and you should now be able to insert a PDF.
The controls will not be visible until you click the container field to select it. You need to select the radio button: Optimize for Interactive Content.
Im going to look at this in the morning, as Im UK based and office closing.
I will come back if Im struggling. Im really appreciating your help.