SumatraPDF not showing up in Windows settings Default Apps

[Friedman10]

SumatraPDF is not showing up in Windows settings Default Apps. Normally an app should appear in the application list to allow users to select the application and then choose which extensions to associate with that application.

[GitHubRulesOK]

That is partly due to Microsoft and Adobe queering the pitch when Edge required it be the Master PDF application and Adobe triggering a retaliation, that now means few others can compete in that Battle of the hashes.

Ensure your installation was correct HOWEVER it may vary per version.
The only RELIABLE way is that Microsoft allows you set associations manually via right click and that is described in

https://www.sumatrapdfreader.org/docs/Set-as-default-pdf-viewer
then you should see it is associated

ODDLY Windows no longer forces its OWN viewer for ePub or XPS as it no longer supports those.

But may still hang onto CHM which it also does not support

[Friedman10]

I get that you can scroll through the alphabetical listing of .extensions and choose, one by one, each file extension you want to associate with SumatraPDF. But generally every Windows application that is installed can be selected by application, rather than by file type, in Default Apps settings. And once you select the application, it lists all (and only) the extensions that application can be used as the default for those extensions.

The other non-Adobe/non-Microsoft PDF editors are listed there in this way to choose what extensions to use them for as the default.

[GitHubRulesOK]

TLDR

BOTTOM LINE
Microsoft apps use a privileged registration model
They are allowed to declare their full list of supported file types to the Settings UI.
Third‑party apps are restricted

you could use

• NirSoft FileTypesMan will show all more than 500 common extension types but not the same easy list as
• DefaultProgramsEditor

During installation SumatraPDF should be REGISTERED for ALL the types it supports (MANY) but Windows decides who is the given default for types already allocated.
So SumatraPDF is NOT the Default for CHM or PDF but should be for most of the many others, that is why it is allocated to Microsofts own XPS format because nothing else was, but MS does not allow it to be dafault for CHM (or PDF) unless the user says so.

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Default Programs Editor

The default programs association for some extensions could not be saved due to a registry permissions issue. This could be a result of previous tweaks by other programs to your settings.

The affected extensions are:
.gif
.jfif
.jpeg
.jpg
.oxps
.pdf
.png
.tif
.tiff
.xps

Try saving these changes to a .reg file, then merging that into the registry.

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BUT that WILL NOT work

“Only the user, through Settings → Default apps, may set defaults for protected file types.”
This is to prevent: Malware hijacking PDF [and those other protected] file handlers

There are thus unethical ways to alter PDF against user choice and Adobe caused issues by enforcing itself over Edges default which was likely the root of all more recent problems because MS check and at one time reset such choices.

File types registered by MS app such as paint will be allocated to SumatraPDF rather than paint
if paint cannot handle such of some that SumatraPDF can
for example paint cannot use heif

Windows before 10 makes it easier to see all the filetypes do not include PDF ! (Darn I now need to see why CBz or Heic are not in this list)