Just adding a few notes here - had a problem of trying to add MuPDF to the Open With list for .pdf files; however, this program was not "installed", I just copied the binaries to C:\bin.

Here is what I did (after several times opening via Open With/Choose another app/More apps/Look for another app on this PC/(look for and choose mupdf.exe)):

• Checked Computer\HKEY_CLASSES_ROOT\.pdf\OpenWithProgids - it had a key named MuPDF
• Checked Computer\HKEY_CLASSES_ROOT\Applications - it had an entry mupdf.exe, that is: Computer\HKEY_CLASSES_ROOT\Applications\mupdf.exe
• Go back to Computer\HKEY_CLASSES_ROOT\.pdf\OpenWithProgids - renamed the MuPDF key to mupdf.exe
• Modified Computer\HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.pdf\OpenWithList, so mupdf.exe is the value of the first entry (the key named a)
• Restart Windows 10 Without Rebooting (restart Windows Explorer from Task Manager) - nothing yet
• Check Computer\HKEY_CLASSES_ROOT\Local Settings\Software\Microsoft\Windows\Shell\MuiCache - only have keys named C:\bin\mupdf.exe.FriendlyAppName;
• Added also: C:\bin\mupdf.exe.ApplicationCompany under Computer\HKEY_CLASSES_ROOT\Local Settings\Software\Microsoft\Windows\Shell\MuiCache

... and at this point, mupdf.exe appeared last in the Open With list, without any restart of Windows Explorer - so, problem solved.

Just adding a few notes here - had a problem of trying to add MuPDF to the Open With list for .pdf files; however, this program was not "installed", I just copied the binaries to C:\bin.

Here is what I did (after several times opening via Open With/Choose another app/More apps/Look for another app on this PC/(look for and choose mupdf.exe)):

• Checked Computer\HKEY_CLASSES_ROOT\.pdf\OpenWithProgids - it had a key named MuPDF
• Checked Computer\HKEY_CLASSES_ROOT\Applications - it had an entry mupdf.exe, that is: Computer\HKEY_CLASSES_ROOT\Applications\mupdf.exe
• Added a key ("folder") SupportedTypes under Computer\HKEY_CLASSES_ROOT\Applications\mupdf.exe - then added a new "String Value" for this key, which I renamed .pdf (and left the value of it blank)
• Go back to Computer\HKEY_CLASSES_ROOT\.pdf\OpenWithProgids - renamed the MuPDF key to mupdf.exe
• Modified Computer\HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.pdf\OpenWithList, so mupdf.exe is the value of the first entry (the key named a)
• Restart Windows 10 Without Rebooting (restart Windows Explorer from Task Manager) - nothing yet
• Check Computer\HKEY_CLASSES_ROOT\Local Settings\Software\Microsoft\Windows\Shell\MuiCache - only have keys named C:\bin\mupdf.exe.FriendlyAppName;
• Added also: C:\bin\mupdf.exe.ApplicationCompany under Computer\HKEY_CLASSES_ROOT\Local Settings\Software\Microsoft\Windows\Shell\MuiCache, with a value of mupdf

... and at this point, mupdf.exe appeared last in the Open With list, without any restart of Windows Explorer - so, problem solved.