If you are on Windows and they are using the regulare Acrobat Reader, this will work:

  system qq(start acrord32 "$pdffile");
[download]

. But if they might have full Acrobat instead of the reader, in which case I'm not sure that will work. Or, theoretically, they might have some other reader.

As long as the filename doesn't have spaces, this will open the file in their configured PDF reader:

  system qq(start $pdffile);
[download]

. Unfortunately there doesn't seem to be a way to do that if the file (or path name) contains spaces (quoting does not help). You could translate it to a DOS short (8.3) name, though.