Originally Posted by CrazEtooN
By download, do you mean view, or do you mean actually save to the device/storage card? If you mean save, you can't without third party software, well you can't save documents anyway. If you can't view them, the suggestion that you may have deleted the attachment viewing service book is definitely a possibility. Check your service books for anything with "attachment" in the name. Other than that, you could always try a battery pull and see if that helps.
Also check under options>advanced options>applications to see if BlackBerry Attachment Service is still there. If not, re-install the OS and you should be set.