Printing | MuseScore

Are you talking about a score that you have entered into MuseScore (on your computer) and you wish to print the individual parts on sheets of paper?
If so, why are you trying to “load” a pdf document? Just print directly from the score (parts) on your computer, to the paper in your printer.

If, on the other hand, you are dealing with the score-sharing website over at musescore.com, for example:
musescore.com/user/30263711/scores/10814989
… it’s best to ask over there (e.g., if you are having trouble downloading).