You send a PDF and get a bounce: "Message size exceeds limit", "Attachment blocked for security reasons", or the email simply never arrives. Mail servers are strict about attachments, and PDFs trip several filters. Here is how to diagnose and fix it.
Cause 1: the file is too large
Most providers cap total message size at around 20 to 25 MB, and attachments grow by about a third when encoded for email. Many company servers set lower limits, often 10 MB.
Fix:
- Compress the PDF. See compress a PDF for email attachments.
- Split it into parts. See how to split a PDF.
- Share a link from cloud storage instead.
Cause 2: the PDF is encrypted
Security gateways cannot scan inside password-protected PDFs, so many organisations quarantine or block them. This catches people sending legitimately protected documents.
Fix:
- Ask the recipient whether their organisation accepts encrypted attachments.
- Use a secure portal or a cloud link with access control instead. See how to share a PDF securely.
- If allowed, send the password through a separate channel and warn the recipient to check quarantine.
Cause 3: the PDF contains JavaScript, attachments or links flagged as risky
PDFs with embedded JavaScript, launch actions or embedded executable files are classic malware vehicles, and filters block them. Even an innocent form with calculation scripts can be flagged. See PDF JavaScript security risks.
Fix: flatten the form or remove scripts before sending, or remove embedded attachments. See how to flatten a PDF.
Cause 4: the PDF was flagged as phishing
PDFs whose main content is a link or QR code to a login page look exactly like phishing. See QR code phishing in PDFs. Legitimate senders can get caught too.
Fix: put real content in the PDF, avoid shortened links, and put important links in the email body as well.
Cause 5: the file name or type looks suspicious
Double extensions (invoice.pdf.exe), very long names or unusual characters trigger filters.
Fix: rename to something simple, such as Invoice-1042.pdf.
Cause 6: the PDF is in a ZIP
Zipped attachments, especially password-protected ZIPs, are blocked by many organisations.
Fix: send the PDF directly, or use a link.
Reading the bounce message
The bounce (non-delivery report) usually contains a code and reason:
- 552 / "message size exceeds": size.
- 550 5.7.x / "policy" or "content rejected": security policy (encryption, macros, file type).
- "Quarantined": delivered to the recipient's quarantine, which they or their IT team can release.
When nothing bounces but nothing arrives
The message may be in the recipient's spam or quarantine. Ask them to check, and to allow your address. For important deadlines, confirm receipt by phone or a separate message.
Takeaway
Keep PDFs small, unencrypted for email (or use a portal when encryption is needed), free of scripts and embedded files, and plainly named. A browser tool like Docento.app can compress, split and flatten files locally before you send them.
