The file may be too large. There is a 5MB file size limit for attachments. A few reasons why...
Sending large attachments can negatively affect deliverability, increasing the chance of a message ending up in a spam folder or being rejected by the recipient's filters.
Email marketing providers tend to not allow attachments at all. Reason being, a 25mb file attached to a blast to 1,000 recipients requires Outstand's servers to process that 25mb file 1,000 times. It's not ideal, efficiency-wise. It monopolizes our server capacity and would affect site performance for all users.
For these reasons, it's recommended to upload the file to a hosting site like DropBox. DropBox can provide you with a link for that file. Check out DropBox's Help Section for a tutorial.
One benefit to using a link instead of an attachment is click tracking. You'll know exactly who clicked the file, where attachments cannot be tracked.
Have a PDF that exceeds the 5MB limit?
SmallPDF.com can cut out the unneeded fat from your PDF file and provide a compressed file without affecting the quality of PDF.