Good question
and the explanations in help : doesn't seem to answer the question unless I missed somthing
Viewing thumbnails in the drawer
When you open several images at the same time or a PDF document with multiple pages in Preview, separate thumbnail images or pages appear in the drawer. (If the PDF has a table of contents, it appears in the drawer.)
You can choose to see both the image and the filename or page number, just the image, or just the filename or page number in the drawer.
Choose Preview > Preferences and click General.
Select an option to either show the image and name, just the name, or just the image in the drawer.
Drag the Thumbnail Size slider to adjust the thumbnail size.
To show or hide the drawer, choose View > Drawer.
To jump to an image or page, click its thumbnail in the drawer.
To set Preview to load all thumbnails automatically (so they don't load dynamically as you scroll through the drawer), choose Preview > Preferences and select "Automatically load all thumbnails."