This sounds like a file association issue in Windows,
Carrara opens the Help in Adobe Acrobat pro for me, ...no issues, and you shouldn’t have any issues in opening this help file if you have either the full version of Acrobat, or the free Acrobat reader installed.
Does Acrobat open the help manual if you double click on the PDF,.? ... or do you need to go to “Open with”
Also, is the icon displayed for the manual the correct one for a PDF. ?
Downloading and installing the free acrobat reader may be the quickest, and simplest way to correct the file association.
running a registry editor should also work.
Hope it helps