I receive mails with attached pdf-files all the time (6 today) and never had this behaviour. But I am using Version 7 (7.2.34648) and have pdfXchange as my default PDF program. Could be a bug in V6 or Acrobat being over sensitive.
Noting to do with acrobat. There is nothing wrong with the file only when it was sent to eM client did it become a problem. I have received other PDFs without an issue but this didn’t work. Losing confidence in this email client. Not to mention eM7 is such a mess for me.