Can anybody explain how to change file extension associations with applications to open them with in Fedora/KDE ? E.g. my PDFs open with Okular and I would like to change the default to Adobe.
There are a few things you can set under System Settings (Workspace Appearance and Behaviour) -> Default Applications, such as the terminal and browser app to use.
With regard to things opened from the KDE file manager, Dolphin, I believe those are contolled from System Settings (Common Appearance and Behaviour) -> File Associations. While I do use fedora+kde, I don't actually use dolphin, so I can't say for sure which is the right one WRT, eg, pdfs, but application->pdf looks promising (there's a search there too).
You will probably also be interested in this dolphin feature.