I want my scrollbar to be visible all the time!

For me I had to set it to Large in order to have it showing without a mouseover.

After digging through many google result pages full of rightful irritation and trying a dozen frustrating suggestions, I succeeded finally on my own by setting "View"-"Display Size" from "Small" to "Standard".
(Adobe Acrobat Reader "Continuous Release Version 2025.001.20531"  on Windows 11.)
Note on some suggestions found in the community: In my Reader version, disable or enable "new" Acrobat Reader makes no difference. Neither does Windows accessibility option "show scrollbars". Some found a "show scrollbar" option in "File"-"Preferences"-"Page Display", but my Reader version hasn't any. Before your Reader version drives you crazy, try "Display size".
@ developers: Please do not hide such essentials as a scrollbar without an explicit option to unhide.

@BenSquig To ensure scrollbars are always visible in Windows, you can adjust a system setting that overrides the default hidden behavior. Navigate to the Windows Settings, then proceed to the Ease of Access section, or Accessibility in more recent versions of Windows, and select Display. Within the Display settings, locate the option labeled "Automatically hide scroll bars in Windows" and toggle it off. 

I myself use the desktop app not the web browser option, so, you could also try the desktop app too.