Your PDF is too large to email

The short answer

Your file is not too big for the limit you were told about. It is too big for the limit after encoding, and nobody mentions that part.

Email cannot carry raw binary data, so every attachment is re-encoded for transit — and that makes it about a third larger on the wire than it is on your disk. A 24 MB PDF is roughly 32 MB by the time it is actually travelling, which is why it bounces off a 25 MB limit that it appears to fit.

Aim for about 70% of whatever limit you were given. Against 25 MB, that means roughly 18 MB.

This is documented, not folklore

Both of the companies that run most of the world's email say so in their own administrator documentation.

Google, on Gmail receiving limits:

"You can receive emails of up to 50 MB (Enterprise Standard) or 70 MB (Enterprise Plus). These values are the limit after encoding, which adds about a 37% increase."

Microsoft, on Exchange Online message limits:

"Messages that are routed outside of the Microsoft datacenters are subject to an additional 33% translation encoding increase, in which case the maximum message size is 112 MB."

The two figures differ slightly because Google is also counting the message headers. The underlying cause is the same: the encoding used for attachments represents every three bytes as four characters, which is a fixed 33% overhead before anything else is added.

And here is the part that turns it from trivia into a rule you can use. Microsoft's own webmail applies the margin for you:

"OWA accounts for the possibility that your message may be subject to the 33% encoding increase and restricts the size of message you can send to 25% less than the configured setting."

If Microsoft builds a 25% safety margin into its own product, that is a reasonable margin for you to use too.

The real limits, from the source

Two of these are worth a second look.

The Microsoft default is 35 MB, not 25 MB. Almost every article on this subject says 25 MB. It is wrong, and it has been wrong for years.

The Outlook mobile app caps messages at 33 MB. The same account, the same mailbox, the same administrator settings — a message you can send from the desktop may be refused by the app on your phone. If something sends from your laptop and fails from your phone, this is why.

The limit that matters is not yours

This is the mistake that wastes the most time.

You check your own provider's limit, you get under it, you send, and it fails anyway — because the message has to be accepted by their mail system, and their employer may allow far less than yours does.

The two systems also fail in opposite ways, which is worth knowing:

• Exchange rejects the message and sends you a non-delivery report. Unpleasant, but you find out immediately.
• Gmail does not reject anything. It removes the attachment and replaces it with a Google Drive link. Your message is delivered and you are told nothing. If the recipient is outside your organisation and the sharing permissions do not suit them, they get a link they cannot open — and from their side it looks like you sent a broken email.

If a document is genuinely important and you are anywhere near the limit, ask the recipient what their limit is. It takes one message and it is the only way to know.

What to do about it

1. Compress to a number, not to a vague setting. "Low, medium, high" tells you nothing about whether you will clear 18 MB. Set the target and read the result.

2. Leave the margin. 70% of the limit. If you were told 25 MB, target 18 MB. If you do not know the limit, assume 25 MB and target 18 MB anyway — that clears every mainstream default including the Outlook mobile app.

3. Check the file opens before you send it. Ten seconds. A compressed scan that arrives unreadable is worse than a message that bounced, because a bounce tells you something went wrong and an unreadable attachment does not.

4. Split it, if it is a long document. A 60-page report is often better sent as two 30-page files than as one heavily degraded file. Splitting a PDF preserves quality completely, and most limits are per message, so two messages doubles your budget.

When compressing is the wrong answer

Some documents cannot be made small enough while remaining readable, and pretending otherwise helps nobody.

A long colour scan is the usual case. If you have 40 pages of scanned colour paper and a 18 MB budget, that is about 450 KB per page, which is workable. If you have 200 pages and the same budget, it is 90 KB per page, and a colour scan at that size is a smear.

At that point the honest answer is: send a link instead. Upload to any file-sharing service and paste the URL. This is exactly what Gmail does automatically when you exceed its limit, and Google is not doing it to be difficult — it is doing it because that is the correct solution to the problem.

The one thing not to do is compress harder and hope. The recipient will open a document they cannot read, and you will not find out until they tell you.

If it is a scan, there is a better lever

Compression is not always where the saving is.

If your PDF is a scan of paper documents, most of its size is the paper: the off-white of the page, the shadow from the book spine, the sensor noise, the faint grey cast that is not carrying any information at all. Compressing that is spending your byte budget on things nobody needs to see.

Rescanning in black-and-white at 200 DPI discards all of it at source, and typically produces a smaller file that is more readable than the compressed colour version — because the bytes go into resolution rather than into recording 256 shades of grey for what is, in the end, black text on white paper.

That takes longer than clicking a button, which is presumably why no other page suggests it. It is still the right answer when the document matters.