"Attachment size exceeds the allowable limit." It's one of the most annoying errors in email, because it usually arrives after you've written the message — and the file in question is a document you have to send. The fix is almost always compression, but there are a couple of non-obvious traps that make people compress a file, try again, and get bounced a second time.
This guide covers getting a PDF under the common email limits, why the real ceiling is lower than the advertised one, and when to stop compressing and send a link instead.
Know your actual limit
The advertised caps for the big providers:
- Gmail — 25 MB per message.
- Outlook / Microsoft 365 — typically 20 MB (and many corporate mail servers are configured lower — 10 MB is common).
- Corporate mail servers — frequently the strictest, and often the real reason a message bounces.
The trap: these limits apply to the encoded message, not your file on disk. Email encodes attachments in a way that inflates them by roughly a third, so a 24 MB PDF can exceed a "25 MB" cap. And if the limit applies to the whole message, several attachments are counted together.
The practical rule: aim well under the stated limit. For a 25 MB cap, target 15–20 MB. For a 10 MB corporate cap, target 5 MB. The headroom is what stops the second bounce.
The steps
Open Blackpdf's Compress PDF tool and drop your file in.
Start with the Recommended preset (Good Quality / Balanced). For the overwhelming majority of files, this alone drops them well under any email limit while leaving the document looking fine.
Check the result against your target — remember to leave headroom. If it's still too big, step up to Extreme (Maximum compression).
On Pro, you can skip the guesswork with Target size: type the size you want and choose MB or kB.
Click Compress PDF, download, and attach.
If it's still too big
Work down this list:
- Grayscale a colour scan. If colour isn't required, converting to grayscale before compressing can cut a lot.
- Send only the pages they need. Often the recipient wants three pages of a forty-page document — extract them.
- Split it into two emails. Split PDF and send in parts — sometimes the cleanest answer for a genuinely huge document.
- Send a link instead. Past a certain size, compressing further just makes an ugly document. Share a link and let the recipient download the full-quality file.
Common questions
Should I zip the PDF instead?
No — it barely helps. A PDF is already a compressed container, so zipping it typically shaves off only a few percent. Reducing the images inside the PDF is what actually cuts the size, which is what a PDF compressor does. Zipping also adds friction for the recipient.
My PDF is under the limit but the email still bounced.
Two likely reasons: the encoded size pushed it over (see above), or the recipient's mail server has a smaller limit than yours. Sending limits and receiving limits are different — the strictest one wins. Compress further, or send a link.
Will compressing make the document look bad?
At normal email targets, rarely. Most documents clear 10–20 MB on the Recommended preset with no visible change. Only when you're forcing a big scan down to a very small size does quality visibly soften — see our full compression guide for the trade-offs.
Can I compress a password-protected PDF to email it?
Not directly. Unlock it first, compress, then re-protect it before sending if it's confidential.
What size should I actually aim for?
If you don't know the recipient's limit, under 10 MB is a safe universal target — it clears virtually every mail server, including strict corporate ones.
Wrap-up
- Drop your PDF into Compress PDF.
- Use Recommended; step up to Extreme only if needed.
- Leave headroom — encoding inflates attachments, so target well under the stated cap (under 10 MB is universally safe).
- Still too big? Grayscale it, send fewer pages, split it, or send a link.
For specific targets, we have step-by-step guides for 5 MB, 1 MB, and 500 KB.