Thank you for your investigation Rob, but even when I have 'Remember last state of the All tools pane when opening documents' ticked, every time I open a document the book mark tab still opens. No matter how many times I close it, when I open the same or a new document, the bookmark tab still opens up.

I wasn't using Acrobat Reader DC very often lately, so after reading latest comments, I also verified that near early mid-December my PC received Acrobat Reader DC MUI v24.005.20320 update, and then I also decided to check Tools pane effect once more.
At first, I only started it via its desktop icon (& without opening any .PDF file), and since Tools pane was not opened, so once again I investigated a little more and inside Preferences -> Documents I saw this option: 'Remember last state of the All tools pane when opening documents'.
I initially thought such option was new and since it was also already enabled, I closed Acrobat Reader DC and then re-opened it while also loading a .PDF document and again I noticed that Tools pane was not displayed too.
Anyway after also checking past comments I figured out that since some versions Adobe has already definitely implemented this idea (see also past comment I added some months ago when I used older v24.002.20687 April 2024 version), and since it seems I'm the only one to have confirmed this at least twice with Acrobat Reader DC MUI (at least with new interface enabled), I also hope this may have helped once again as an additional implicit suggestion to also consider upgrading to Acrobat Reader DC (and also because of Adobe own official statement "If you are still using Adobe Reader 11.x or other older versions, please note that those are no more supported by Adobe. Try out the latest, Acrobat Reader DC.
In case your concern is not addressed in the latest version, please report the issue against Acrobat Reader DC.").

This is the 3rd most voted feature request and it's nearly been up for 2 years. There are also similar requests as the 4th (submitted in 2020) and 6th most voted for features, and yet Adobe is refusing to listen to its user community and implement such a basic request as hiding the tool or bookmark panel by default. Come on Adobe, sort it out!!!

Acrobat Reader opens the Tools Pane EVERY TIME I open a document, and I always immediately close it. Why can't you remember our preference and CLOSE the TOOLS pane?!?!?

I tried every one of the 5 suggestions David Do recommended in comment below. The issue persists. Ridiculous that more than 5 preference choices could be the root cause.

Also implemented Bhawna suggestion of Path in Acrobat : Hamburger > Preferences > Documents > Restore last view settings on reopening the documents. The issue persists.

Please fix and update.

I'm unsure about versions preceding last current one v24.002.20687 I've started re-using again, but as of now inside Acrobat Reader DC MUI Preferences -> Documents there's a 'Remember last state of the All Tools pane when opening documents' that works fine and since after I enabled it allowed me to avoid seeing that pane every time I open Acrobat Reader DC.

It is December already, and I hope I can get the feature early. I use Reader to display PDFs on a public booth computer, and I don't want to see the pane every time I open Acrobat for PDF. Thank you!

Hi Dylan,

Thank you for your valuable feedback. We are pleased to inform that we have taken this concern into consideration and will be deploying this feature in December.

This privilege is already supported in Acrobat and we will now be supporting the same in Reader as well.

Path in Acrobat : Hamburger > Preferences > Documents > Restore last view settings on reopening the documents.

Best,
Bhawna.

This currently already exists if I'm not mistaken, though it is a little complicated to achieve as you more or less, need to deactivate everything that might trigger the tools pane. So, there are a few different settings that may need to be set all together to achieve the expected view consistently, but as far as I'm aware (and only included in case you may not have already tried setting them):

Go to View > Show/Hide > deselect "Tools Pane" (I presume this is what you mean by having to close it every time, but on the chance that you meant you more simply have to click that small arrow to have the tools pane slide it away, then using the settings to hide it should persist between program restarts.

If not...

Review these other options that may trigger the tools pane to appear:

Edit > Preferences >> General (Categories) >>> Under "Basic Tools" uncheck the box for "Show quick actions on text selection"

Edit > Preferences >> Documents (Categories) >>> Under "Open Settings" set "Allow documents to hide the menu bar, toolbars, and window controls"
as well as "Restore last view settings when reopening documents" to active by checking the appropriate box.

Additionally consider these settings:

Edit > Preferences >> Full Screen (Categories) >>> under "Full Screen Navigation" unselect "Show navigation bar"

Edit > Preferences >> Reviewing (Categories) >>> Uncheck the box for "Automatically Open Toolbars for Reviews"

If by chance, you would prefer to simply have a reader and never find yourself needing the Tools Pane at all, there is a comment that contains a workaround to eliminating it completely, found in a similarly suggested thread found here:
https://acrobat.uservoice.com/forums/926812-acrobat-reader-for-windows-and-mac/suggestions/40077811-hide-not-collapse-right-tool-pane-completely#comments

Which describes steps from

https://community.adobe.com/t5/acrobat-reader/how-do-i-remove-the-right-hand-pane-in-reader-dc-locked/td-p/7021654?page=1