Outlook Attachment Limit: Fit PDFs Under 20 MB

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.

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.

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.

All PDF tools

Browse every PDFThin tool in one place.

Email-ready compression

Shrink PDFs to Gmail/Outlook limits with quality control.

Split by size

Split large PDFs into size-safe parts.

Organize PDF

Merge, rotate, delete, reorder, and split pages.

All PDF guides

Browse every step-by-step PDF guide.

Compress PDF for email (complete)

Step-by-step workflow for Gmail 25MB and Outlook 20MB limits.

Reduce scanned PDF size

Keep text readable while shrinking scanned documents.

Split PDF by size

Create parts that stay under a target MB limit.

Rotate PDF pages online

Fix page orientation for sideways scans or photos.