You click attach, pick the file, watch the progress climb — then 'attachment failed' or 'file rejected by server'. The PDF won't go.
What's actually going wrong
File too large. Email provider's limit exceeded. Most common cause. Security filter. PDF contains content the mail server blocks (scripts, certain forms). Network upload failure. Connection dropped mid-upload. Corrupt file. Mail server scanned the file and found it broken.
The quick fix
Most attachment failures are size. Compress PDF in Flint and try again — typically resolves it.
If the file is small and still fails, restart your email client to clear any stuck upload state. Try again. Often fixes it.
If that didn't work
For security-filter rejections, the file's content is being flagged. Convert via PDF to JPG and back to PDF — the new file is structurally simple, image-based, and rarely triggers filters.
For corporate environments with strict policies, ask IT what's allowed. Some companies block all PDFs with form fields, or all PDFs above a certain size. The fix is policy or alternative sharing.
Prevent it next time
Compress every PDF before attaching. Don't email PDFs with embedded scripts. Use cloud sharing for files over 20MB. And keep your email client updated.
FAQ
Why does my small PDF still fail to attach?
Security filter or upload glitch. Restart the email client. If it still fails, try converting through Flint to produce a simpler file that filters don't flag.
What's the largest PDF I can attach reliably?
Under 25MB on Gmail, under 20MB on most other providers. Above 10MB, expect occasional failures with corporate spam filters.
Can I attach a PDF to Outlook differently if it fails in Gmail?
Different providers have different filters. A file rejected by one might go through another. Compression and conversion usually solve cross-provider issues at the source.
Will my recipient's email block the PDF?
Possibly — recipient policies vary. If you suspect their server rejects PDFs, share via cloud link instead. Works for any size and any policy.
Most email attachment failures fix with compression in Flint. For policy issues, share via link.