Sometimes a corrupted PDF prevents your message from sending. In the past, we've seen Amazon Seller Central withhold sending messages with problematic attachments like this, even though your message my show as sent in your account.
To confirm that your attachment is corrupted, you may try to send a test message to yourself. If the test message does not deliver to you, or if you cannot open the downloaded attachment, this indicates there is an issue with your attachment.
To fix this issue, please remove the attachment from your message in the Communications Center, and then re-upload. To remove the attachment, navigate to the "Review and Edit Message" screen, scroll down to the Attachments section under the text editor section, click the trash can next to the attachment, then click "Save & Send Test" or "Save & Complete". Once the attachment is re-uploaded, monitor your Sent page in your Seller Central account to ensure this message resumes sending as expected.
Also, you might consider re-sizing your attachment to ensure the file size isn’t conflicting with the message in some way. You can shrink PDF files quickly using the free service at smallpdf.com, or you can shrink your images using the free service at picresize.com. These will typically not reduce the resolution quality.
If you need assistance, you may email us the PDF and we can attach it for you (firstname.lastname@example.org).