Hi, It seems you are currently not having a mail server. So, I propose you the following solution. I'll give you a mail id. From that mail_id, mail will be send at the specified date/time to the list of people you want. Whenever you want the attachment to be updated you can just send a mail to the given mail_id with the new attachment. From that point on, all the send mail will have the new attachment.
You can send a mail with subject "New Attachment" for updating the attachment, "Send now" as subject for sending mail at non specified times, "Add new recipient(s)" for adding new recipients and so on..
If you have a linux server then everything is fine. Otherwise a server space needs to be rented and it'll cost 10USD/year.
Thanks and Regards,