It's under the View menu.
In all the help articles and even in your respone notthing is said that this is for Arobat Pro only (not standard)
I feel this is lousy -- MS Word has had a compare tool free with basice version for decades
How to I let someone at Adobe who is responsible for the help articles and product features know what I am saying.
Myself, and many others, have let people know in the past that this feature is only available in the Pro version. It's also clearly stated here: Plans and pricing | Adobe Acrobat DC