@Ander3232

Correct Firefox, Foxit, Others and even Adobe have had to devote teams to overcome the way MS keep moving UAC settings, the silly point is the encryption for User preferences is fairly simple to circumvent so MS retrigger(s) Edge as dominated PDF default. A clean reset Adobe is about 150 registry settings ! but primary entry can be seen with
REG QUERY HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.pdf\UserChoice that should show MSEDGE as UAC application

The best way to reset is USER select ANY PDF press ALT/Enter OR right click PROPERTIES then change the "open with" then ignore Edge and other old assignments including ignore any SumatraPDF icons as potential old entries keep move down to select always open AND LOOK for another app on PC then pick the EXE you want e.g. either in %localappdata% or %programfiles% sumatrapdf e.g. which ever has the desired sumatrapdf.exe now if you check you should see similar to

> REG QUERY HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.pdf\UserChoice

HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.pdf\UserChoice
    ProgId    REG_SZ    SumatraPDF.pdf
    Hash    REG_SZ    ugfTZgIpvT8=

the problem is the Hash can change each time so as example resetting

ProgId    REG_SZ    MSEdgePDF
Hash    REG_SZ    **2YAsaliqJuU=**

ProgId    REG_SZ    SumatraPDF.pdf
Hash    REG_SZ    ugfTZgIpvT8=

ProgId    REG_SZ    MSEdgePDF
Hash    REG_SZ    **Tn9SNtvdjd4=**

thus varies each time

there is an exe and powershell script that aims to reset by hashing and it is interesting that the SFTA tool uses SumatraPDF as an example (this is NOT a proposal or endorsement just illustration of typical use)

Register Application + Set File Type Association
SFTA.exe --reg "C:\SumatraPDF.exe" ".PDF"
SFTA.exe --reg "C:\SumatraPDF.exe" ".PDF" "CustomProgramId"

Register Application + Set Protocol Association
SFTA.exe --reg "C:\SumatraPDF.exe" "http"

Unregister Application
SFTA.exe --unreg "C:\SumatraPDF.exe" ".PDF"
SFTA.exe --unreg "CustomProgramId" ".PDF"*

Simple command was SFTA.exe Applications\SumatraPDF.exe .pdf but you need to experiment.
Most recent (22 days ago) PowerShell version https://github.com/DanysysTeam/PS-SFTA
older exe https://danysys.com/set-file-type-association-default-application-command-line-windows-10-userchoice-hash-internal-method/

The best way to reset is USER select ANY PDF press ALT/Enter OR right click PROPERTIES then change the "open with" then ignore Edge and other old assignments including ignore any SumatraPDF icons as potential old entries keep move down to select always open AND LOOK for another app on PC then pick the EXE you want...

Yup, tried that, of course. Several times. Result: That particular PDF opened in Sumatra—but the default app (Edge) remained unchanged, even though I selected "Always use this app to open PDF files".

The good news is, I think I've solved the problem (until MS gets busy again, LOL)! I switched from the 64-bit Sumatra to the 32-bit, and Windows has accepted it as the default PDF app. The 64-bit world has always seemed to have some lingering flakiness; I've solved various problems by backpedaling on various other 64-bit apps (on various PCs). Whatever works though, right? 🙋🏽‍♂️u