Open your Portfolio in Acrobat Pro and make sure you're looking at the Navigator, not at one individual member document. Press CTRL-J or Command-J to open the JS console. Press ENTER once to wake it up, then clear the messages using the trashcan icon in the bottom right corner. Type this into the console, make sure your cursor is still at the end of the line, and press ENTER or Ctrl+Return:
this.collection.initialView = 'Details';
The reply will be "Details". Close the console and save the file. Now whenever it opens you'll get the file details view.
To undo the change and set the navigator as the default again, use this line instead:
this.collection.initialView = 'Custom';