Recently, presumably upon an update to Word 2016, the following phenomenon started.
I usually open documents from Explorer or "Recent" on the taskbar.
Until a few weeks ago, files would open within the same instance of Word, whether it was already running or not. This remains the case with Excel, PowerPoint and other programs. But no longer with Word. Every file opens in a separate instance. If I open three files together, three new instances of Word begin. When I close a file, I need to exit the instance too. The only way to open a file in the same instance is Alt-F, O and then a laborious clicking process through directory folders to find the most recent version of each file. By which time I have probably lost my train of thought.


Since I have a relatively new and clean Windows and Office installation, I was expecting to see that other users had encountered and reported the same issue. But a Google search does not show any remedy for the issue. Hence this post.
I would assume that some Registry setting needs to be edited to return to normal.
Can anyone suggest a fix?

This link might be of interest to you:

With Word 2016 can you Show all documents in the same window?

https://answers.microsoft.com/en-us/...4-471e40af3dd7

Thank you Moonshine for alerting me to this post. I noticed that change when I upgraded from 2003 to 2016 a few years back.
The challenge I now have is parallel instances of Word running for each document I open, not merely separate windows of the same instance. Whenever I open a document (other than through Alt-F O) the blue "Word Office 365" splash screen appears followed by the new instance of Word. When the document is closed (Ctrl-W or Alt-F C), the empty instance remains: Word does not switch to the next open document. This bug only occurs with Word, not with other MS Office programs that I use.

Have you tried repairing the Office installation (via Windows Control Panel > Programs > Programs & Features > Microsoft Office (version) > Change > Repair)?

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

Thank you Paul for reminding me of what I used to do prior to version 2016 upon encountering this type of issue. Repair performed, I tried opening three *.docx files 20 seconds apart, to allow time for pagination. Each opened in a fresh instance of Word, just as before. So Repair does not appear to fix this issue, which would be logical if the setting change had arrived within a recent update.

Try a minute apart. Still same problem? My guess is that it is not fully open by the time you start the next document.

Otherwise, you can report this problem using the smiley face in the upper right of the Word window and/or uservoice.
UserVoice Word for Windows

Thank you Charles for being diagnostic! I have now tried one minute, one hour, one day and one week apart but without impact on the result: a new instance always opens. Thank you for recommending the smiley face which I shall try and will report back here on progress.