I found the tip on Microsoft windows 10 help, first and foremost. Secondly, Adobe acrobat is the only pdf reader I use (and thus what the file was originally associating to). My problem lied in the fact that even when the .jar -was- associating to java (happened after the 11 trillionth install), it still wasn't running. There are quite a few posts on Windows 10 help with people complaining about the same thing "I upgraded from 8 to 10 and now my .jar doesn't work" etc, so I could only assume it was a WIN10 thing.
My point is that for this to be a Windows 10 bug you'd have to show indication of it someone purposefully or randomly *giving* the JAR association to Adobe Acrobat. There is no indication of that. That means that Acrobat had to take it. And that's, apparently, not unheard of:https://forums.adobe.com/thread/1156228
When "typical" fixes for miss/re-associations are not holding, then something else is almost always amiss.
The program you used to FIX the issue even spells it out:
The root cause for the problem above is, that a program has stolen the .jar association.
That's the issue that JarFix is correcting. It's not fixing a Windows 10 bug.
At any rate, this behaviour is atypical. Given the the popularity of Acrobat (and Win 10) we'd be seeing reports of this all the time. We aren't.
But, as mentioned, if Acrobat takes the association again (possibly with an update or a potential re-install should you need it) I'd look for a more permanent solution than the jarfix method. Like uninstalling Acrobat, running a registry cleaner, reinstalling Java, then reinstalling Acrobat (and checking its preferences on install to make sure it never attempt to take the association to begin with).