Title says it all.
Thumbnails are shown by default in multi-page PDFs.
In single page PDFs though, the thumbnail sidebar is hidden by default. I have to manually enable it every time.
I copy pages around frequently enough, so it would be very nice to have the thumbnail sidebar always available.
In normal everyday use, OPT+CMD+1 and OPT+CMD+2 from the View Menu are my friends.
Once again, I learn something new on this forum. 
Thank you.
Do you know if there’s a way to always show the thumbnail sidebar?
I mean, leave it on by default at all times, no matter how many pages the document has.
Nope, sorry, that’s why I know the keyboard shortcut by heart. 
I didn’t try it but as I was sure there was a preferences toggle for this I went searching. So, perhaps try this (if this is still supported on Monterey):
(and the same advice at How to have sidebar with thumbnails by de… - Apple Community)
defaults write com.apple.Preview PVPDFSuppressSidebarOnOpening -bool false
If you use Keyboard Maestro, you might be able to use it to get this to work. I do something similar with the annotation toolbar in Preview. I use an “if-then” conditional action in Keyboard Maestro. When Preview opens, the macro looks for the menu item that activates the annotation toolbar. That menu item is only there when the toolbar is not visible. It then types the keystroke to activate the menu item if it is available.
I tried it, but the entry is no longer there since macOS Sierra.
In fact, I found an update for this plist entry mod, but didn’t work for me.
https://piggott.us/2017/09/mac-os-x-sierra-displaying-thumbnails-in-preview-by-default/
<key>PVSidebarViewModeForNewDocuments</key><integer>1</integer>
Mine was already set to “1”. I tried to set it to “0” and even “2” to see what would happen, but Preview presents the same behaviour: Thumbnails for multipage PDF, no thumbnail for single page PDF.
The value reverts back to “1” when I close and open Preview again.
I have an old license of KM. It really never clicked for me.
I don’t have it installed right now. In fact, I don’t even know if its compatible with the current macOS 
Thanks for the input though.
I think I’ll have to resign myself to use the keyboard shortcut to display the thumbnails, at least for now (⌥+⌘+2).
i use an action script one but yourps looks more simpler. Def using it
try KM, create a macro once and you’ll automate small annoyances like this
