converting pdf-files

I cannot convert pdf-files to MuseScore3 (it also didn't work in MuseScore 2 for the last 6 month)
Even if the program says it succeded and can be downloaded, I cannot open it with the MuseScore program.
Long time ago I succeed with the converting in MuseScore 2, but that doesn’t work any longer. Could it be a problem, that PDF is upgraded since?
Brigitta Pranov