Two things to try
1. Start Reader. Maybe it will say "Reader is not the default app to open files" and offer to fix it. Does it work? If not, try
2. Start Reader. Use Help > Repair installation. Does it work? If not, try
3. Start Reader. Go to Edit > Preferences. Click General tab. Near the bottom, click Select As Default PDF Handler (a button).
Thanks so much! The first suggestion worked perfectly. Much appreciated.
Thanks for letting us know, glad to hear it.
Can anyone tell me how to *permanently* stop Edge from setting itself as default for PDF when MS updates are installed ? I keep having to do this over and over. Microsoft, of course, offer ZERO help.