Just bought and installed the DC-9 in FS24 and can't find the manual.
The JF installer creates a link to a Documentation folder in the installed aircraft, but doesn't create the Documentation folder itself!
The answer is you have to install the FS20 version which does have the Documents folder.
Luckily I still had FS20 installed.
The Just Flight FS24 installer needs repackaging to include the manuals.