Just migrated from 18.04 to 20.04. pdf files were associated with document viewer OK until I installed Vivaldi browser. Since that clicking a pdf file will open it in Vivaldi. It is possible to force it - but the option to make it default just is not there. in the Settings - Default Applications - there is no option for "document" of any kind at all
my computer is Dell Inspiron and the installation was made "from scratch".