How can I configure Reader XI to open all documents in the same window instead of as individual windows?
The functionality you are looking for is not available in the older version of Adobe Reader. You need to update it to the newer version which is Adobe Reader DC
You may check the preferences settings whether the box highlighted in the below image is selected or not?
Launch application> Edit >Preferences>General
As you do want to open the multiple documents in same window, so make sure "Open documents as new tabs in the same window" is selected.
Hope this will help.
This option is not avaliable on the latets 2021 version of DC
This feature needs to come back ASAP! Why was it taken away?
Install the latest version of Acrobat Reader DC.