Campaigns not sending attachments

We are facing issues while sending emails having attachments via campaigns.

1. Create campaign

2. Use Mail form of Usage Internet Mail

3. Comm Channel : Email with Activities

4. Select Target group

5. Attach a PDF with Camapign

6. Release and execute the campaign.

7. Emails received by the contacts do not have the attchments.

SOST checked and the attachments do not exist there as well.

What can be the issue and how to fix it? Please help.