Reference
Compress a PDF for Outlook
Microsoft 365 caps attachments at 10 MB by default. That is far below Gmail, and it is the most common reason a send fails from a work account.
| Service or procedure | Maximum size | Source |
|---|---|---|
| Microsoft 365 / Exchange (work account) | 10 MB | Microsoft |
Checked on 2026-10-17.
A limit your employer can change
The 10 MB is a default, not a hard rule. A Microsoft 365 administrator can lower it to 5 MB or raise it to 150 MB. If you don't know your real ceiling, aim for 8 MB: that gets through everywhere.
The limit applies after encoding, so to the weight carried rather than the size shown in your file explorer. Always leave a margin.
Bringing the PDF under the threshold
- Open Compress PDF and drop the document in.
- Pick a 8 MB target so you stay under the cap whatever it is.
- Download the file and attach it in Outlook.
Everything runs in the browser. For a work document — a contract, a client file, an accounting record — that keeps sensitive information from passing through a third-party service.
The other routes on Outlook's side
- OneDrive or SharePoint: Outlook offers to send a link instead of the file, with no size limit, but the recipient needs permission to open it.
- Split the send into two messages with Split PDF, useful when the recipient is outside your organisation.
- Check the recipient's receiving limit first: it is often theirs that blocks, not yours.
FAQ
What is the attachment limit in Outlook?
10 MB by default on Microsoft 365 and Exchange. Your administrator may have changed it either way.
Why does Outlook reject a 9 MB file?
Encoding adds about a third to the weight actually carried. Aim for 8 MB to keep a margin.
How do I find my company's exact limit?
Your IT team can tell you. In the meantime, 8 MB is a safe target in almost every configuration.
Is my file uploaded anywhere?
No. The compression runs on your device and the document is sent to no server.