>I just tried the setting changed the Zoom to 100% (from Automatic) and the setting appears take effect (except for files that I've got opened at the time -- closing, then re-opening will make the preference setting take effect).
>What I've got installed reports as Acrobat Reader DC 2015.009.20077
>
>Not using Windows 8.1 however -- using Windows 7
>Tried on Windows 10 -- the setting appears to take effect (change in setting doesn't affect any files already opened -- closing them then re-opening the new preference does change).

Thanks, my version is now exactly like yours.

Here is, attached, the JPG image showing the preference. That setting is saved. When I open the document back, it is saved. However, the document always opens at 174%.