Edit by moderator: this post and its replies have been detached from mail images so readers can focus on troubleshooting this specific problem with .pdf attachments.I don't know if this is the same thing, but what happened to me during the week was:
I have been sending some invoices as PDF attachments by email for years. Since Mail inserts attachments rather than er, attaching them, I can see my PDF right there in the body of the email, pretty much full size, before clicking Send. No one ever had a problem with this.
This week, same technique, same systems, my attached PDF displayed as a teenytiny postage stamp. No I didn't compress it first. I tried two different PDF-makers with same result. Eventually I converted the invoice to a JPG ( ! ) just to get it away by email.
I don't like weird things like this. Things should WORK or NOT WORK, not HALF WORK, Dammit!