Send a PDF that will not fit through your mail server

The message bounces, or worse, it silently does not arrive. The file is 22 MB and the limit is 25 MB, so the arithmetic looked fine. It was not, and the reason is a detail of how email has worked since the early nineties.

Why 22 MB does not fit in a 25 MB limit

Email bodies are text. Binary attachments are therefore encoded into text before transmission, and the standard encoding is base64, which represents every three bytes as four printable characters. That is a 33 percent expansion before you count MIME headers and line breaks.

So a 22 MB PDF becomes roughly 29 MB on the wire. Against a 25 MB limit it fails, and the error message rarely explains why.

Two more complications:

• The limit that matters is the smaller of the two. Your provider might allow 25 MB while the recipient's corporate gateway caps at 10 MB. You cannot see their limit and you will often not get a useful bounce.
• Intermediate systems shrink the window further. Security scanners, archiving appliances, and mailing lists each impose their own ceilings.

A practical rule: keep attachments under about 15 MB on disk if you want them to arrive reliably in the general case, and under 7 MB if the recipient is a large organisation.

Fix one: reduce what is actually big

Open the question of why the file is large before compressing blindly. For almost every oversized PDF, the answer is images.

A text-only 60-page report is typically a couple of megabytes. A 10-page scanned document can be 40 MB, because each page is a full-colour photograph stored at scanner resolution. Fonts add tens or hundreds of kilobytes; images add tens of megabytes.

That points at the levers worth pulling, in order of effect:

1. Downsample image resolution. Scans are frequently stored at 600 dpi when 300 dpi is indistinguishable in print and 150 dpi is fine on screen. Halving the resolution quarters the pixel count.
2. Re-encode images appropriately. Photographic content belongs in JPEG; line art and text scans compress far better as monochrome or indexed colour. A scanned form saved as a full-colour photograph is the classic waste case.
3. Convert colour scans to greyscale when the original was black ink on white paper. This alone often removes half the size.
4. Drop redundant embedded data. Duplicate image objects, unused embedded fonts, old incremental revisions, and thumbnails accumulate silently.

The compression tool exposes these choices rather than hiding them behind a single slider, and the quality-preserving guide explains how far you can push each one before the result starts to look degraded.

Fix two: send less document

The most effective size reduction is not compression. It is deciding that the recipient does not need all 300 pages.

If someone asked about the Q3 figures, extract the Q3 section with the split tool and send four pages. The email arrives instantly, the recipient finds what they needed without scrolling, and you have disclosed substantially less context than the full document would have.

This is the option people skip because it requires a judgement rather than a button, and it is almost always the right one. The splitting walkthrough covers extracting ranges cleanly.

Fix three: split into numbered parts

For a document that genuinely must arrive whole — a bound report, a complete filing — split it into sequential parts and send them as separate messages.

Make it usable for the recipient:

• Name the files so order is obvious: report-part-1-of-4.pdf.
• Split at chapter or section boundaries, not arbitrary page counts.
• Say in the message how many parts to expect, so a missing one is noticed.
• Send them close together; parts arriving hours apart get lost.

If your mail system is happier with a single attachment, the local ZIP packaging tool will bundle the parts into one archive on your machine. Note that ZIP compresses an already-compressed PDF hardly at all — this is about tidiness and count, not size.

Fix four: link instead of attach, carefully

Sharing services solve the size problem by moving the file to someone else's storage. That is a fine trade for a public brochure and a poor one for a confidential document, because it reintroduces exactly the third-party copy that the rest of this site exists to avoid.

If you go this route, prefer storage your organisation already controls, set an expiry, restrict access to named recipients rather than anyone with the link, and do not use a consumer file-transfer site for regulated material.

Prevent it at the source

Most oversized PDFs are born that way and were avoidable:

• Scan at 300 dpi, not 600, and in greyscale unless the colour carries meaning.
• Export from your word processor or design tool using a screen or print preset rather than the maximum-quality preset.
• Insert images at the size they will be displayed instead of dropping in a 12-megapixel photo and scaling it down on the page.
• Avoid printing to PDF from a viewer, which frequently rasterises a perfectly good text document into page images.

Ten seconds of attention at export time saves the entire compress-split-resend cycle later.