What does "Adobe cannot decrypt this document" mean?

I received a link to download a songbook as a bonus for purchasing an album -- I have Adobe Reader 7.0, and the download is an Adobe file. When I go to open it, I get that message. Mine is not the most recent version of Adobe, but it is fully updated and I download PDFs from all sorts of sources, and haven't any problem at all. I contacted the vendor, who are not only generally unhelpful and a bit nasty, but also not the sharpest knives in the drawer, told me the problem was that I wasn't opening it with Adobe (?) Here's the link to download it and the link is to Adobe X -- I know of another person who opened the file with Adobe 9, I think the file is corrupt in some way, but I don't know, could you help me? And I'm on dial up, so committing to downloading Adobe X is a big one, but I tried several times, and it doesn't work for me. Thanks for any help...