We have one user for which the "Open documents as new tabs in the same window" preference does not work properly.
Reader has been uninstalled/reinstalled multiple times. The preference works properly at first, then after a short period, the docs start opening as separate windows, even thought the preference is still set to use tabs.
The user previously had Acrobat Pro installed, during which time the problem first surfaced. After uninstalling Pro and installing Reader, the problem persists.
Hope you are doing well and sorry for the trouble. As described the "Open documents as new tabs in the same window" preference does not work properly for a user and you have already tried uninstalling and reinstalling the application with no luck .