How to email a PDF that's too big

The message bounces, or sits in the outbox refusing to go, and you're left guessing how much smaller the file needs to be. Most oversized PDFs are oversized for one boring reason and shrink dramatically once you deal with it. The catch is that the limit you're aiming at is lower than the number your provider advertises.

The limit is smaller than the number on the tin

Gmail says 25 MB. Outlook and most corporate Exchange servers say 20 MB, though administrators routinely set it lower; 10 MB is common. The figure that applies is lower than the advertised one, for two reasons.

First, attachments are encoded for transmission in a format that inflates them by roughly a third. A 20 MB file arrives at the server as something closer to 27 MB, and it's the encoded size that gets measured. Your effective ceiling is about three-quarters of the stated limit.

Second, the limit that applies is the lowest one anywhere along the path. You may send happily from a 25 MB service to a recipient whose employer caps incoming mail at 10 MB, and the failure notice, if one arrives at all, comes from their server, not yours.

A practical target: get under 10 MB for general use, and under 5 MB if you're writing to a large organisation, a government department, or anyone whose systems you can't ask about. Under 2 MB is safe everywhere.

Step one: find out what's actually big

Before compressing anything, take a moment to work out which kind of file you have, because the right fix differs completely.

If the document contains photographs or scanned pages, they are almost certainly the entire problem. A phone photo is 3–8 MB; a colour scan at 300 DPI is similar. Ten scanned pages is a 40 MB PDF, and no amount of clever packing changes that: the images have to come down.

If it is text and vector graphics (a report exported from Word, an invoice from an accounting system) and it's still enormous, something structural is wrong. Compression will barely help, because there's no image weight to remove. Why is my PDF so large? walks through the usual culprits: a font embedded whole rather than subset, an invisible full-page image behind the text, or accumulated editing history from an editor that appends changes instead of rewriting the file.

A quick diagnostic: divide the file size by the page count. Above roughly a megabyte per page, you're looking at images. Well under that, with a large total, points at something structural.

Step two: compress to a target

For the image case, compression is the whole answer, and it usually works better than people expect. The mechanism is downsampling: each embedded image is re-encoded at a resolution appropriate to the size it is actually displayed at, rather than the resolution the camera or scanner produced. A photo displayed six centimetres wide doesn't need four thousand pixels across, and reducing it to what the page uses is invisible on screen and in print.

Guessing a quality setting and re-checking the size is tedious. It's easier to name the ceiling and let the tool find the setting. Compress PDF takes a target, 1 MB, 500 KB or 200 KB, and searches for the highest quality that fits underneath it. For emailing a scanned document, 1 MB is usually generous and still perfectly readable.

Check the result before sending, particularly the smallest type on the densest page. Compression trades quality for size, and the point at which that becomes visible depends on the document.

Step three: send less document

Often the recipient doesn't need all of it. Sending the three relevant pages of a sixty-page report is faster for both of you and solves the size problem as a side effect. Extract pages pulls out what matters. If the document really must arrive in full, Split PDF can divide it into parts small enough to send across two or three messages.

Splitting is a last resort: multi-part attachments get separated, arrive out of order, and one part usually goes missing. Number the parts clearly in the filenames and say in the message how many there are.

When it won't shrink enough

Some documents genuinely can't get under a strict limit while remaining useful: a hundred-page colour scan, an architectural drawing set, a high-resolution photo book. Compressing those to 5 MB produces a file nobody can read, which is worse than not sending it.

At that point email is the wrong transport, and the alternatives are ordinary: a shared link from whatever cloud storage your organisation already uses, or a file-transfer service. Both come with the same caveat: you're putting the document on somebody else's server, and for a confidential file that's a different decision from attaching it to an email. Check whether the link is public or restricted, and whether it expires.

If the document is sensitive enough to make that uncomfortable, password-protect it before uploading and send the password by a different route. That keeps the contents unreadable to the storage provider and to anyone who guesses the link.

Before you send

Emailing a document means it leaves your control, which makes this a good moment for a habit that has nothing to do with size: check what else is in the file. Author names, the software that produced it, editing history and creation dates all travel with it, and a document prepared for one recipient often still carries another's details. Strip the metadata, or run the privacy scanner if the recipient is someone you'd rather not tell more than you meant to.