Importing Incomplete PDFs

Of all the sheet music PDFs I've been downloading and trying to import, the ones that go through are always incomplete, sounding somewhat like the song to start and then losing it completely and I'm left with a bunch of empty measures at the end after the song flounders 30 seconds in. I'm just trying to write vocal arrangements. I can't write piano parts or accurate vocal parts so I have to import PDFs just to do what I want, but that doesn't work.