Outlook Attachment Limit: Fit PDFs Under 20 MB
Outlook commonly caps attachments at 20 MB for internet accounts. This guide shows how to compress or split PDFs to deliver without errors.
Step 1: Target 20 MB or less
Use the Outlook 20 MB preset to target the typical limit. PDFThin optimizes first to preserve layout and clarity.
If you attach multiple files, choose an even smaller custom target to keep the total size under the cap.
Step 2: Reduce scan-heavy PDFs
Scanned documents are the most common reason Outlook attachments fail. Smart quality can reduce image size while keeping text readable.
If you can rescan, use grayscale and lower DPI to keep output smaller.
Step 3: Split oversized files
If the PDF still exceeds 20 MB, switch to Split by size. You will get parts under the limit in one ZIP. Send the parts in order and label them clearly in the email body.Step 4: Test before sending
Attach the new file and confirm Outlook shows no warnings. If it does, lower the target by a few MB and retry.
This quick check avoids repeated send failures.
FAQ
Is Outlook.com different from Outlook desktop?
Outlook.com allows 25 MB, but many Outlook desktop accounts enforce 20 MB. Use the stricter limit to be safe.
Why do attachments fail even under 20 MB?
Encoding overhead can inflate message size. Try 18 MB or split the PDF into smaller parts.
Can I send a ZIP instead?
Yes, but some corporate filters block ZIP files. PDF parts are often more accepted than ZIPs.
Outlook-friendly tips
Plan for overhead
Base64 encoding can add 30 percent overhead. Targeting 18-19 MB improves reliability.
This is especially useful when sending multiple attachments.
Keep parts organized
When splitting, attach parts in sequence and mention how many parts the recipient should expect.
This small note avoids confusion and helps recipients recombine files later.