Can not open .pdf or .doc files in mail

Every time I receive an email with a pdf or .doc file attached I can never open it as I get a message telling me that the file was not recognised or that the file is invalid. Even if I select to open using Adobe for the pdf I get the same.

Any help gratefully received. I've always had this problem and get round it by viewing these emails online instead. But it's not as convenient of course.