Preparing documents for email attachments

Most mail servers cap attachments somewhere between ten and twenty-five megabytes, and the cap that matters is the recipient's, not yours. A message can leave your outbox happily and bounce at the other end.

Check the real number first

Encoding overhead means an attachment arrives larger than it left. Usually by around a third, because attachments are encoded as text for transport. A twenty megabyte file against a twenty-five megabyte limit is not the comfortable margin it looks like.

Fix the cause, not the symptom

If the document is a scan, compress it. That is the case compression is genuinely good at, and a 300 dpi colour scan of a text document will often drop by eighty per cent with no meaningful loss of legibility.

If the document is text with a few images, compression will do little. The size is coming from somewhere else. Check whether the file has accumulated revision history, and whether images inside it are at camera resolution when they are displayed postcard-sized.

Split rather than zip

Zipping a PDF saves almost nothing. PDFs are already compressed internally, so the archive is roughly the same size with an extra step for the recipient. It also trips spam filters at some organisations and gets stripped entirely at others.

Splitting a long document into logical parts is usually better received than either. Three emails with a clear part number in each subject line is more useful to the person filing them than one enormous message.

The link option

For anything genuinely large, a link to shared storage is the right answer, with two caveats worth remembering. Links expire, and documents behind them change. If the file is a contract or anything that may be referred to later, send the file itself, because the recipient's copy is the record.

Before it goes

Two habits worth building: clear the metadata, and open the final file yourself before attaching it. Roughly one in ten problems at this stage is not size at all. It is the wrong version, or a page that failed to render, which you would have caught in five seconds by looking.

Why the arriving file is bigger than the one you sent

Attachments are encoded as text for transport, which inflates them by roughly a third. A twenty-megabyte file arrives as something closer to twenty-seven.

This is why an attachment can pass your outbound limit and fail at the other end. The number that matters is the recipient's, after encoding, and you generally cannot see either.

Treat any limit you are told about as roughly a quarter smaller than it sounds.

Reducing size in order of what it costs you

Free: clear the metadata, flatten filled forms, and resave the file to discard revision history. None changes how the document looks.

Cheap: if the document is a scan, compress it. A 300 dpi colour scan of black text is carrying several times more information than anyone will use.

Expensive: dropping resolution or quality far enough to soften text. Do this only when the document merely needs to be legible.

Wrong: zipping. PDFs are already compressed internally, so the archive is barely smaller, adds a step, and gets stripped by some organisations entirely.

Splitting sensibly

If a document has to go in parts, split it on meaning rather than arbitrary page counts. Chapters, sections, appendices.

Put the part number in the subject line, not just the filename, because that is what the recipient will be scanning in their inbox. Three clearly labelled messages are easier to file than one enormous one.

When to send a link instead

For anything genuinely large, shared storage is the right answer, with two caveats.

Links expire, and documents behind them change. If the file is a contract, an invoice, or anything that may be referred to later, send the file itself. The recipient's copy is the record, and a link is not a copy.

The check that catches most problems

Open the final file, the one you are about to attach, not the one in your editor, and look at it.

Roughly one problem in ten at this stage is not size at all. It is the wrong version, a page that failed to export, or a form still showing placeholder text. Five seconds of looking catches all three.

Sending to organisations with strict filters

Some sectors, finance, healthcare, government, run mail filters considerably more aggressive than the norm. Attachments carrying embedded files or active content are commonly stripped or quarantined without notifying anyone.

If you are sending to one of those and the document is important, sanitise it first to remove scripts and attachments, keep it to a plain PDF, and ask the recipient to confirm arrival rather than assuming silence means success.