The BEST ANSWER I have seen so far is at how can I keep tool pane and thumbnails open as a default?‌ . That response advises using (1) F4 to get the "Page Thumbnails Pane" open or closed. After you have pressed F4 to open the Page Thumbnails Pane, (2) ALSO click EDIT, PREFERENCES, DOCUMENTS and CHECK the box beside "Restore last view settings when reopening documents". Click OK / Apply to save the check box change. (3) Then be sure you SAVE the current file you have F4'ed your preferred view. In my case, I ALWAYS need Page Thumbnails open for the files I update. I hope this helps you as much as it did me.
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