Hi Bill,
We understand that it is important that your settings are set up permanently when you configure them. If you're setting up a different default program when opening PDF files and it always reverts back to Microsoft Edge, it is possible that there is an issue with you r current user profile. For us to isolate the issue, please create a new administrator account so that we can verify if the issue will occur on a new user profile.
If the issue persists, it is best to report your concern in the Feedback Hub. You can refer to the reply of Leviozar Gam to know how to submit an issue in the Feedback Hub. If you've already reported this issue there, we highly recommend that you post the link of your submission here so that other community member can upvote it.
Thanks.