This one is making me crazy. Using macOS High Sierra 10.13.4, FMPA 16.0.05.500. Two files - one the container field behaves as expected, 2nd file it does not. Same format settings on both. I use container fields frequently and have never seen this. When control-click on container field to bring up option menu, two completely different results. For ease of explanation attached are screen shots. File 1 behaves as all the rest do and is correct. File 2 does not. Both files have been in use for years and are unfortunately large and mission critical .So this is an unexpected result.
Thanks in advance for any advice or insight.
Interactive containers don't work in portals. I didn't consider this as a possible cause because:
a) you didn't tell us it was in a portal
b) you said that "both files have been in use for years". I read that as this being a previously working layout that had stopped working--my mistake here.
You'll need to consider other design options, such as a "view" button in the portal row that causes the PDF to show in an interactive container outside the portal or an interactive container outside of the portal with scirpting/triggers such at inserting/dropping a PDF into it causes the correct update of a container field in the portal's table.