Adobe After Effects is a different program. If it took the original PDF file and added some kind of graphics effects, ti would explain the user sending you an ".aom" file, but it could also have been an error on the original poster's part. You can try forcing the issue by changing the name of the file to ".pdf", but Acrobat files have signature strings embedded in the front end which have to be there for an Adobe Reader or Acrobat application to open them. So it might not work but is worth trying.

A