That'll happen until the registry values are corrected.
I'm not sure why changing the default app via Open With isn't fixing the registry values, though. When I try the same on a just-updated Windows 11 system and fresh FoxIt Reader install, it updates both UserChoice and UserChoiceLatest, and everything works as expected in Opus.
A little while after first installing FoxIt, there was a notification from Windows about an app "damaging" the file type PDF registration and it being reset to MS Edge as a result (Microsoft accusing others of what they are doing themselves, sigh), but that affected both Opus and File Explorer, and changing the association back to FoxIt using Open With worked OK in both programs and then stuck.
Unfortunately, Microsoft have intentionally made it difficult to change the default web browser or .pdf registry entries directly, going to the extent of adding a new driver to Windows that blocks modifications to the relevant registry data even when permissions should allow editing. There are ways to disable the driver, as well as to turn off the new UserChoiceLatest functionality, but both require some command-line and registry editing. (New Windows driver blocks software that changes default web browser - Techzine Global and https://kolbi.cz/blog/2025/04/20/userchoicelatest-microsofts-new-protection-for-file-type-associations/ have more detail if you're technically inclined.)
The easiest workaround is to override the PDF double-click action within Opus:
- Settings / File Types
- System File Types / PDF (or type ".pdf" into the search field under the list).
- Commands tab, then Events tab under that.
- Edit the Left double-click event.
- Set it to run FoxIt:
"C:\PROGRAM FILES (X86)\FOXIT SOFTWARE\FOXIT PDF READER\FOXITPDFREADER.EXE" "%1"
(If you're not sure of the program path to FoxIt on your system, it'll probably also be in the Actions tab under the Open event. That's where I got my example command.)
Then it doesn't matter what's in the registry. The double-click event is part of the Opus configuration and will override the registry data. (Within Opus. It doesn't affect the rest of the system.)
