I'm not totally clear what you want here, but offer the following responses:
1. If you want to be able to display both a pdf and a jpg in the same field, then no you can't. A container field can only hold one item per record. Record 1 can hold a pdf and record 2 can hold a jpg, etc.
2. You can configure each instance of a container field separately, either to optimise for image display or interactive display, and you can have both versions on the same layout if you wish. Interactive will give you the ability to display the contents of a multi-page pdf by scrolling. Image will only display the front page. Either will display a jpg the same way.
3. You mention portal, so perhaps you are displaying a number of related container records of various types. If the portal row is larger enough for a good sized field, you could just configure a field to Interactive and it will display both equally well—with a scroll bar if needed. Typically, however, portal fields are small so you might consider placing a popover button on the portal row (or configuring a small image version of the container field as a popover button) and then placing a much larger interactive version of the field on the popover.
The third option is the most applicable to me. However, the portal that contains the container fields is not displaying any records. What is your suggestion ?