When double clicking a file in Windows Explorer where the path is longer than 260 characters does not work as default. But I have been able to create a workaround that enable this.

If I type the following in cmd.exe:
AcroRd32.exe "C:\Very long path\file.pdf" then that will fail.

If I enable "Enable Win32 long paths" in Group Policy and try the same command, then it works. This means that long paths are supported when it is enabled in Windows.

I created a bat-file like this:
set str=%1
set str=%str:\?=%
AcroRd32.exe %str%

This will remove \?\ from the first argument and launch AcroRd32.exe with the trimmed argument.

I changed the registry key HKEYCLASSESROOT\AcroExch.Document.DC\shell\Read\command\ to use this bat-file. And now it works!

It looks like the problem is that Acrobat Reader does not understand if the path start with \?\ but Windows will add this to the start of long filenames.

Information about \?: https://docs.microsoft.com/en-gb/windows/win32/fileio/naming-a-file?redirectedfrom=MSDN

NOTE! I'm writing backslash, backslash, questionmark, backslash. But after posting it only shows \?\