As discussed in this thread, these files (opened attachments, opened files from browser, or any other directly opened file without saving them) cannot be deleted an easy way (this is Microsoft oversight and the have to fix this, even though they don't).
What you can do:
a) Clear Word (or another Office application) data in Android application's settings. This will delete all those files (it will not touch any other files on your device or in cloud) and you will have to log into Microsoft account again. This will not delete
the application itself, it will just clear it's internal data.
b) If you have root permissions (and I doubt that), you can directly find those files in any file manager.
c) Pester Microsoft support and ask them to file a bug report regarding this issue.
About your re-installation thingy - you can download them back from either Google Play or Samsung Store.