I have windows 10 and for years have had my Acrobat v.9 be the default pdf program. Recently for some reason it has changed and even though, when I go to the default programs in the control panel; choose by file type; .pdf - drop down on right - choose Adobe Acrobat 9 - it returns to Microsoft Edge. No matter what I do I cannot change it.
Acrobat 9 is a very old version, it's unsupported, and not compatible with any current operating system. Problems, especially when it comes to integration with the OS, are to be expected.