Why won't my PDF attach to an email?

You attach the PDF, press send, and it bounces. Or worse, it appears to send and the recipient never receives it.

The cause is nearly always size, and there are two facts about email size limits that explain most of the confusion.

Fact one: attachments get bigger in transit

Email was designed to carry text. Binary files — anything that is not plain characters — have to be encoded into text to travel through it, and the standard encoding, base64, makes the data about 33% larger.

So a 20 MB PDF arrives at the server as roughly 27 MB. A 24 MB file becomes about 32 MB and is rejected by a 25 MB limit, which is exactly the moment people conclude that the limit is a lie.

The practical rule: your real ceiling is about three quarters of the stated limit. For a 25 MB service, aim for 18 MB and stop worrying.

Fact two: the recipient's limit is the one that matters

Your provider accepting the message only gets it as far as their server. It then has to be accepted by the receiving server, and that limit may be much lower.

Common ceilings, before encoding:

That last row is where most mystery failures live. A perfectly ordinary 12 MB attachment leaves your machine, is accepted by your provider, and is silently dropped by a corporate filter at the far end. Sometimes you get a bounce. Sometimes you do not.

Making the file fit

Three approaches, in the order worth trying them.

Compress it. Compress PDF re-encodes images and rebuilds the file structure. On a document with photographs or scans this routinely takes 60-80% off, and text stays sharp because text is not what is being compressed.

If it barely shrinks, your PDF was already efficient — that is a real answer, not a failure, and it means the size is genuine content rather than waste.

Reduce the page count. If the file is long rather than heavy, SmartShrink puts the content of several sparse pages onto one sheet without shrinking the text to nothing. Useful for reports with a lot of white space.

Split it. Split PDF divides the document into parts small enough to send separately. Less elegant, but it always works, and for a contract with distinct sections it is often what the recipient wanted anyway.

What not to do

Do not zip a PDF. A PDF is already compressed internally, so zipping saves a percent or two — and many corporate mail filters block .zip attachments outright, so you have made the file marginally smaller and much more likely to be rejected.

Do not rename the extension. Sending a .pdf as .txt to dodge a filter fails: filters inspect content, not names, and if it does get through the recipient cannot open it.

Do not screenshot the pages. A folder of PNGs is usually larger than the PDF was, and the text stops being selectable.

The alternative to attaching at all

For anything above about 20 MB, a link is the better answer regardless of limits. Upload to whatever storage you and the recipient share and send the link.

This also avoids the quiet problem with large attachments: every recipient of a forwarded thread carries a copy, so a 20 MB file sent to eight people becomes 160 MB across the organisation's mailboxes, plus another copy every time somebody replies to all.

A working order

1. Check the size. Under 10 MB, it is almost certainly not the problem.
2. Over 10 MB, compress it and check again.
3. Still large, and it is a long document, try reducing the page count.
4. Still too big, split it or send a link.
5. If it is under every limit and still failing, the attachment is not the cause — check the address and whether the recipient's filter blocks PDFs at all.

When the size is fine and it still fails

If your file is comfortably under every limit and the message still does not arrive, size is not the problem. Three other causes, in order of how often they turn out to be it.

The recipient's filter blocks the type. Some organisations quarantine PDFs from outside senders entirely, because a PDF can carry scripts. You will usually get no bounce, because quarantine is not rejection.

The PDF contains something a scanner dislikes. Embedded JavaScript, an attached file inside the PDF, or an embedded form action can all trigger a filter even when a plain PDF would pass. Re-saving the document through a tool that rewrites it — compressing it, for instance — often strips whatever tripped the scanner.

The whole message is too big, not just the attachment. A long thread with several previous attachments quoted underneath adds up. Starting a fresh message rather than replying to a long chain sometimes fixes a send that has been failing all afternoon.

Sending several documents at once

The instinct is to attach five files. Two better options.

Combine them. Merge PDF makes one document, which is one attachment, one download and one thing for the recipient to keep track of. For related material — a claim and its receipts, a report and its appendices — this is simply better.

Send a link. Above about 20 MB, or above three or four files, a link avoids all of it.

What to avoid is a ZIP of PDFs, for the reasons above: barely smaller, and much more likely to be blocked.

A note on forwarding

Large attachments do not just cost you once. Every recipient of a forwarded thread stores a copy, and every reply-all carries it again.

For anything above a few megabytes going to a group, a link is the considerate choice as well as the reliable one — and it has the side benefit that you can update the document afterwards without sending a correction to everybody.