Outlook's stated cap covers the encoded payload. After base64 overhead, the safe raw file size is around 14MB for consumer Outlook and 20MB for Microsoft 365 business. The recipient's Exchange server can be much stricter. Aim for under 10MB when sending to any corporate address.
Compress a PDF under 10MB hits the corporate-safe target reliably.
Outlook rejects your PDF with an unhelpful error, or worse, sends it and the recipient never sees it. Both failure modes come from the same source: the number Microsoft prints in its help articles measures the wrong thing, and even that number is only a starting point once corporate mail servers get involved.
The Outlook numbers, translated
Microsoft publishes different caps for different Outlook products. Each one measures the attachment after base64 encoding, not the raw file size on your disk. Since base64 adds roughly 37 percent overhead, the number you can actually attach is smaller.
Raw file. Stated cap is 20MB encoded. Anything larger triggers a OneDrive suggestion.
Raw file. Stated cap is 33MB encoded. The most generous Outlook tier.
Raw file. Corporate defaults commonly land at 10MB encoded, sometimes lower.
The numbers in the middle look generous. In practice, they matter only if the recipient runs the same tier. When you email someone whose company hosts its own Exchange server or applies a security gateway on top, none of those consumer numbers apply.
Why Exchange is the real ceiling
Outlook is a mail client. Exchange is the server behind it. Every attachment sent through Outlook eventually crosses an Exchange server, and Exchange is where administrators set organizational limits. Those limits usually beat whatever Microsoft printed as the "consumer" cap.
Message size limit. Set on individual mailboxes. Default 10MB on Exchange 2019, 25MB on Exchange Online.
Transport rule. Overrides mailbox limits with an org-wide policy. Frequently set to 10MB by IT departments to keep storage costs down.
Connector limit. Applied at the border where mail enters and leaves the org. Often the tightest number in the chain and the hardest to see from the outside.
Because these settings stack, a message can pass your outbound Exchange server, cross the internet, hit the recipient's inbound Exchange, and be rejected there without anyone telling you. Corporate mail environments often do not bounce the message back; they log it and drop it. The sender assumes delivery.
Fix one: compress before you send
Compression is the cleanest fix because it changes nothing about how the recipient receives the file. The attachment is still an attachment, still arrives with a preview icon, still saves to the usual folder. Nothing about the workflow changes.
Pick a size target that matches the recipient
Under 10MB for corporate. Under 18MB for consumer Gmail or Yahoo. Under 20MB for Microsoft 365 business.
Run a size-targeted compressor
Instead of guessing a quality preset, use a tool that iterates until the file fits. Our under-10MB compressor and for-email compressor both work this way.
Verify quality before sending
Open the compressed file, zoom to actual size on pages with the smallest text. If it reads clearly, attach and send.
Compress vs OneDrive link
Outlook offers a shortcut when your file is over the limit: upload to OneDrive and send a link instead. It works, but the recipient's experience changes in ways that matter.
Delivered as a real attachment
- Arrives with a paperclip icon and preview
- Downloads without a sign-in prompt
- Survives forwards, replies, and archives
- Works forever, independent of your OneDrive
- No permission dialog on the recipient's side
Changes the recipient workflow
- Requires a Microsoft account sign-in for external recipients
- Corporate spam filters sometimes flag as suspicious
- Breaks if you move or delete the file
- Bandwidth-hungry recipients may skip the link
- Historical thread search does not surface the content
OneDrive is the right choice for genuinely huge files that cannot compress, or for internal sharing within a company that already uses Microsoft 365. Anything else, compression wins.
Compression targets by recipient
Match the target to who is receiving. Aim tighter if you are unsure.
- Outlook.com personal, iCloud, YahooUnder 14MB
- Microsoft 365 business (org to org)Under 20MB
- Any corporate Exchange (self-hosted)Under 10MB
- Government, hospital, universityUnder 10MB
- Legacy mail gatewaysUnder 7MB
- Unknown recipient tierUnder 10MB
When compression will not be enough
Some PDFs cannot fit under 10MB while remaining useful. Photograph-heavy portfolios, high-resolution scans of long documents, and layout-critical creative work all resist compression. Three fallbacks work in that case.
Split the file
Two 8MB emails almost always beat one 20MB email that bounces. Split the PDF into "part 1 of 2" and "part 2 of 2" with our Split PDF tool, then send in sequence. Common for legal deliveries and design proofs.
Use a link, but pick the right one
OneDrive is the default inside Microsoft 365. SharePoint links work identically inside an org. For external delivery, WeTransfer and Swiss Transfer produce time-limited download links that do not require a sign-in, which most recipients prefer over cloud-provider links.
Ask before assuming
One quick message asking "can your inbox take a 15MB attachment?" saves hours when the answer is no. This is especially true when the recipient sits behind a corporate gateway you have no way to inspect.
Habits that keep files under the ceiling
Export smaller from Office. When saving as PDF from Word or PowerPoint, pick "minimum size (publishing online)". Files can drop by 60 percent with no visible change.
Compress images first. If the source has embedded photos, reduce their resolution in Word or PowerPoint before exporting. That prevents the PDF from carrying full-resolution originals it will never display.
Skip embedded fonts you do not need. Advanced PDF export options let you subset fonts. Full-font embedding can add megabytes to text-only documents.
Aim below the ceiling. Landing at 8MB when the corporate limit is 10MB survives more gateways than landing at 9.8MB.
Compress under 10MB in your browser
Corporate-safe target for any Outlook or Exchange recipient. Nothing uploaded, nothing tracked.
Common questions
Details that come up after the first bounce.
01What is Outlook's real attachment size limit?
Outlook.com consumer accounts advertise 20MB and effectively deliver files up to about 14MB of raw size after base64 overhead. Microsoft 365 business accounts advertise 33MB and deliver up to about 20MB raw. Corporate Exchange servers typically cap tighter, often at 10MB or below.
02Why do some Outlook attachments bounce and others do not?
The recipient's server applies its own cap on the way in. Even if your Outlook accepts and sends a 20MB attachment, the recipient's Exchange server can reject it at their border. Corporate environments often do this silently, without returning a bounce message.
03Does OneDrive help?
OneDrive links bypass the attachment cap but change the delivery. Recipients see a link instead of a paperclip, external recipients often need to sign in with a Microsoft account, and corporate mail gateways sometimes strip link previews. For genuinely huge files it is a valid fallback. For files that can compress under 20MB, compression is cleaner.
04How much can compression shrink a PDF?
Scans and image-heavy PDFs commonly drop 60 to 90 percent without visible quality loss. Text-based PDFs exported from Word or Google Docs shrink far less, sometimes only 10 to 15 percent, because they were already small. If a text-only PDF is over 10MB, something unusual is inside, and re-exporting from the source with better settings usually helps.
05What is the safe target for corporate recipients?
Under 10MB. Corporate Exchange servers, security appliances, and legacy gateways cluster around a 10 to 15MB inbound ceiling. Aim tighter when you can, and expect silent rejection when you cannot.
06Does PDFslime upload the file to compress it?
No. Compression runs entirely in your browser using WebAssembly. Files are read into local memory and never sent to a server. Close the tab and the data is gone.