I am using kubuntu 20.04,(in Joplin Dev environment) I try to import pdf to Joplin Dektop
firstly there was not direct option to import pdf
after clicking other application there is no response
What exactly are you trying to do? If you are just trying to attach a pdf to a note then you just need to copy & paste/drag & drop and it will attach to the note.
There isn't a native importer for PDF -> MD notes so you would need to convert that first somehow.