Unfortunately, opening and then closing attached PDFs does not change anything when in the latest version of the app. They still don't show up as thumbnails in the center pane of the latest version on the Mac, and only as PDF-file symbols in the right pane of the screen (need to double click an open even on the Mac to be able to see/read them). Please have a look at the two screenshots attached: #1 shows the look in the latest app version on the Mac with thumbnails only as grey symbols. #2 shows the same view in the legacy version, where all thumbnails are shown as visual representations of the attached file.