Microsoft sets different limits for different setups. Outlook allows 20 MB when it sends through an Outlook.com or Gmail account, a Microsoft 365 mailbox defaults to 35 MB to send and 36 MB to receive, and an Exchange server left at its default allows 10 MB. Exchange measures the message after encoding, which adds about a third. Aim for under 10 MB, and nearer 7 MB, for any corporate address you cannot check.
Compress PDF under 10 MB targets that size and stops at the first setting that fits.
Outlook rejects your PDF with an unhelpful error, or worse, sends it and the recipient never sees it. Both come from the same place: several limits apply along the way, Microsoft prints different ones for different products, and servers measure the message after it has grown in transit.
The Outlook numbers, translated
Microsoft's Outlook help gives two figures: 20 MB for internet accounts such as Outlook.com and Gmail, and a 10 MB default for Exchange accounts, both covering the message and its attachments together. Attachments are base64 encoded for transit, which makes them about a third larger than the file on your disk.
Microsoft 365 mailboxes run on Exchange Online, which defaults to 35 MB for sending and 36 MB for receiving. Administrators can set anything from 1 MB to 150 MB, and messages that leave Microsoft's datacenters carry the encoding increase on top.
Microsoft's limit when Outlook sends through an internet account, message and attachments together.
Default for sending, 36 MB to receive. Administrators can set 1 to 150 MB.
Raw file under the 10 MB default, which Microsoft measures after encoding.
The middle figure looks generous, but it is only your side. When the recipient's company runs its own Exchange server or a security gateway, their limit applies to your message on the way in, and it can be much lower.
Why Exchange is the real ceiling
Outlook is a mail client. Exchange is the server behind most corporate mailboxes, and it is where administrators set organizational limits. Microsoft's guidance there is that Base64 encoding adds about 33 percent, so a limit has to be set a third higher than the message size it is meant to allow. Its own example is a 64 MB setting that admits messages of about 48 MB.
The organisation limit covers all mail in and out, and defaults to 10 MB each way on Exchange Server 2016 and 2019.
Connector limits apply where mail enters and leaves. The receive connector defaults to 36 MB and the send connector to 10 MB.
Mailbox limits and mail flow rules can set a tighter figure for particular people or attachments. Neither is set by default, and the most restrictive limit anywhere in the chain wins.
Because these settings stack, a message can pass your outbound server, cross the internet, and be rejected at the recipient's. Exchange normally sends a non-delivery report, but a security gateway in front of it may drop an oversized message without one, and the sender assumes delivery.
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 10 MB, or nearer 7 MB, for a corporate address you cannot check. Under 18 MB for a Gmail recipient, which covers the Gmail side of the problem. Microsoft 365 recipients on default settings accept more, but you rarely know their settings.
Run a size-targeted compressor
Instead of guessing a quality preset, use a tool that tries stronger settings until the file fits. Compress PDF under 10 MB tidies the file structure, recompresses the images step by step from 200 ppi, and stops at the first stage under 10 MB.
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
Changes the recipient workflow
- May ask external recipients to sign in or verify by email
- Corporate spam filters sometimes flag as suspicious
- Breaks if you move or delete the file
- Historical thread search does not surface the content
OneDrive is the right choice for files too large to 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.
- Sending from an Outlook.com accountUnder 20 MB with the message
- Personal Gmail recipientUnder 18 MB
- Exchange Server on its defaultAbout 7.5 MB
- Government, hospital, universityUnder 10 MB
- Unknown corporate recipientAbout 7 MB
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 recipients can open without signing in.
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, choose Minimum size (publishing online), which lowers image resolution in the output.
Compress pictures in the source. Reducing embedded photos in Word or PowerPoint before exporting stops the PDF carrying full-resolution originals it will never display.
Subset fonts. Advanced PDF export options can embed only the characters a document uses, where full embedding adds whole typefaces to a text-only file.
Leave headroom. A file at 7 MB clears a strict 10 MB Exchange default after encoding; one at 9.8 MB does not.
Compress under 10MB in your browser
The target for Outlook and Exchange recipients. Runs on your device; nothing is uploaded.
Common questions
Details that come up after the first bounce.
01What is Outlook's real attachment size limit?
It depends on the account you send from. Microsoft gives 20 MB for internet accounts such as Outlook.com and Gmail and a 10 MB default for Exchange accounts, message included; Microsoft 365 mailboxes default to 35 MB. In classic Outlook for Windows, File, then Account Settings lists the type of each account, which tells you which figure applies.
02Why do some Outlook attachments bounce and others do not?
The recipient's server applies its own cap on the way in, measured after encoding. Even if your Outlook sends a 20MB attachment, a recipient on a default Exchange server will refuse it. When a bounce does come back, the code 5.3.4 in it means the message was over a size limit, not that the address was wrong.
03Does OneDrive help?
OneDrive links bypass the attachment cap but change the delivery. Recipients see a link instead of a paperclip, external recipients may be asked to sign in or verify by email, and corporate mail gateways sometimes strip link previews. For files too large to compress it is a valid fallback. For files that compress under the recipient's limit, compression is cleaner.
04How much can compression shrink a PDF?
Scans and image-heavy PDFs shrink the most, because nearly all of their weight is pictures that can be resampled. Text-based PDFs exported from Word or Google Docs shrink much less, because there is little to remove. 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?
About 7MB. Exchange Server's default limit of 10MB is measured after encoding, which leaves roughly 7.5MB for the file, and a security appliance in front of it can be stricter. If the PDF will not go that low, split it or send a link.
06Does PDFslime upload the file to compress it?
No. Compression runs entirely in your browser. Files are read into local memory and never sent to a server. Close the tab and the data is gone.