Okay, it looks like this is an old layout that didn't get updated when some app functionality changed about a year ago. To fix that manually, you can click the Edit Layout button, select Edit Document Viewer, click its options button, select Expand to Full Screen and make sure Show Initially is off. Then click Edit Layout and select Save.
Alternatlvely, you can delete this layout, then click Help > Utilities > Load Default Layouts to recreate it, then edit its details to clear the Show Document Viewer action again.