import pdf

Importing a pdf is a bit like trying to construct a bicycle from a picture of one. If your pdf is clear enough and the music is simple enough you may get reasonable results. Musescore's pdf import uses Audiveris optical recognition using some default settings. Some people have better results using the Audiveris app directly and fiddling with its settings. Others report better results with other software. Others - me included - don't bother with trying to convert a pdf using software but instead enter the music directly into MuseScore, it is generally easier and more certain than trying to spot and fix the inaccuracies introduced using software. No doubt the software approach will improve in the future, but at present I think it is still too unreliable.

It is an issue with musesccore.com and the Audiveris used there, not an issue with the MuseScore editor, so not dealt with here