PDF won't attach to an email: causes and fixes

A PDF that will not attach to an email: where the line actually falls

The situation looks simple: the file is smaller than the stated limit, and the message will not go. Or it goes and never arrives. The cause is almost always one of two things mail services do not talk about: an attachment weighs more inside a message than it does on disk, and there is more than one limit.

Twenty megabytes of file is almost twenty-seven megabytes of message

Email is a text protocol. A binary file cannot travel through it directly, so an attachment is re-encoded into text: every three bytes of the original become four characters. The increase is exactly one third, and on top of it come headers and line breaks.

The convenient arithmetic: multiply the file size by 1.37 and you have the approximate size of the message.

Hence the practical conclusion: a 25-megabyte limit reliably admits a file of about eighteen. A 20-megabyte limit admits about fourteen.

The encoding is not a quirk of one provider but a property of the protocol itself. Sending a finished result to an email address, a feature built into this service, works the same way: the file is encoded by the same method, and the attachment limit there is its own, around fifteen megabytes.

There are two limits, and the smaller one wins

The second source of confusion. Your mail service caps the outgoing message, the recipient's server caps the incoming one, and those two numbers are unrelated.

Personal mail services sit around twenty to twenty-five megabytes. Corporate servers are configured by administrators, and there you find anything: ten megabytes, five, sometimes less. Organisations that handle a lot of paperwork often trim attachments deliberately to keep storage under control.

This case has a distinctive signature. The message leaves your sent folder with no error, and a few minutes later a bounce arrives from somebody else's server. It almost always states both the reason and the receiving side's exact ceiling. That message is worth reading in full: it is the only place their limit is ever disclosed to you.

If nobody named a requirement, ten megabytes is a sensible target. That value goes through practically everywhere.

How to get under the limit

1. Look at the actual file size in its properties rather than guessing. 2. Multiply by 1.37 and compare with your mail service's limit. 3. If it does not fit, open compress-pdf and squeeze the document. 4. Check the new size: with default settings, compression takes a visible share off scans and almost nothing off text documents. 5. If the file is still too big afterwards, cut it with split-pdf and send it in parts. 6. Send the message to yourself before sending it to the addressee; that way you see the bounce before they do.

When compression does not help

Compression is powerless in one specific case: the file is already optimal. That happens with documents exported straight from an editor, and with files that have been compressed once already.

The service is honest about it and returns the original file untouched, saying so in its own message: no compression pass produced anything smaller than the input. That is information, not a failure. The guarantee here is simple: the result is never heavier than what you uploaded.

Three routes remain, and they are not equivalent.

Split the document into parts. Always works, but the recipient gets several messages and has to reassemble them. Name the parts so the order reads at a glance: part 1 of 3, part 2 of 3.

Remove what does not belong. If the file holds pages the addressee does not need, remove-pages takes their weight away entirely rather than partially. That often beats any compression.

Remove the colour. A grayscale scan weighs roughly half of a colour one, because three colour channels are replaced by one brightness channel. pdf-to-grayscale does exactly that. Bear in mind that it redraws the pages as images, so the text layer disappears: irrelevant for a document a human will read, important for one that will be searched.

A link instead of an attachment: when it fits

Large mail services offer to replace an oversized attachment with a cloud link. The temptation to accept is strong, but the choice has a cost worth thinking about beforehand.

A link lives by its own rules. Access rights are configured separately, and by default they are frequently open to anyone holding the link. The file stays available after the message has been read, and stays in your cloud until you delete it. A recipient who forwards the message forwards the access with it.

For a work document, a contract, or a package of personal data that is usually unacceptable, and the right path is to fit under the limit rather than route around it. For a draft, a deck or a set of photographs, a link is more convenient than an attachment and creates no problems.

It is also worth remembering that some organisations trim external cloud links at the same gateway where they trim attachments. In that case the link fails exactly as the file did.

What gets mistaken for a size limit

Some refusals look like a size problem and are caused by something else.

A file with active content inside. A PDF can carry scripts and embedded files, and mail gateways strip such documents entirely or replace them with a placeholder. Size has nothing to do with it. Rebuilding the document through any processing pass usually settles it, because the result is assembled anew.

An extension that does not match the content. If the file is called `.pdf` and holds an archive or a picture, some mail filters react to the mismatch between the file's signature and its name.

The total size of the message rather than of one attachment. Five files of four megabytes give twenty megabytes before encoding and about twenty-seven after. The limit counts the whole message, so a bundle of attachments is better assembled into one PDF with merge-pdf, or spread across several messages.

The check before sending

There is a trick that saves a round of correspondence. Send the message to yourself first, with a second address of yours on a different mail service in copy.

In a minute you see three things: whether the message cleared your outgoing limit, whether it reached somebody else's server, and how the attachment looks on the receiving end. That last one is no trifle: some gateways rename attachments or wrap them in an archive, and the addressee gets something other than what you sent.