When a PDF is too large to email

The message that an attachment is too large is one of the last genuinely confusing errors in everyday computing, because the number in the error rarely matches the number your provider advertises, and because the limit that stops you is often not yours at all. Two mechanics explain almost every case. First, attachments are encoded into text before they travel, and that encoding adds roughly a third to the size, so a 20 MB attachment leaves your outbox as about 27 MB. Second, both ends enforce a limit, and the smaller of the two wins — a corporate mail server capped at 10 MB will bounce a file that your own provider accepted happily. Anything you plan should assume the effective ceiling is well below the advertised one.

The numbers that actually apply

Gmail advertises 25 MB, and because of encoding overhead the real limit for the file on disk is closer to 18 MB; above that it offers a Drive link instead of an attachment. Outlook and Microsoft 365 default to 20 MB for internet mail, though administrators routinely lower it and some set it as low as 10 MB. Corporate Exchange installations very commonly sit at 10 MB. Many older or self-hosted servers still use 10 MB, and a few university and government systems use 5 MB. The practical planning number, if you do not know the recipient's setup, is 10 MB — a file under that will pass essentially everywhere, and a file between 10 and 18 MB is a coin toss decided by a server you cannot see. This is also why a file can send successfully to one colleague and bounce for another at a different company an hour later.

Compress first, because it usually just works

If the PDF contains scans or photographs, compression is the whole answer and it is not close. A twelve-page scanned document we measured went from 17.69 MB to 1.49 MB, and a seven-page slide deck from 6.55 MB to 1.87 MB — both comfortably under every limit listed above, and both still perfectly readable on screen. Do this before considering anything more elaborate, because it keeps the document as a single attachment that arrives in the recipient's inbox rather than as a link they have to click, sign in to, and remember to fetch before it expires. Check the result at full zoom on the busiest page before you send it, and if the smallest text still reads cleanly you are finished.

When compression is not enough

Some files resist. A three-hundred-page text document is large because it is long, not because it is wasteful, and compression will barely touch it. A CAD drawing or a design proof is mostly vector geometry, which is already compact. For these, splitting is the honest next step: send chapters, or send the section the recipient actually needs and offer the rest on request. Recipients generally prefer this — three attachments of 4 MB are easier to handle than one of 12 MB, and nobody wants the whole manual when they asked about section four. If the document must arrive whole and cannot shrink, a shared link is the correct tool rather than a workaround.

Links, and their two failure modes

Cloud storage links solve the size problem and introduce two of their own. The first is permissions: a link shared as "anyone in my organisation" is invisible to an external recipient, and this is the single most common reason a shared file does not open. Test it by opening the link in a private browsing window, which is the closest quick approximation to being someone else. The second is durability. Links expire, folders get reorganised, and accounts get closed, so a link is a poor way to deliver anything that needs to exist in a year — a contract, an invoice, a report that will be referenced later. For those, get the file small enough to attach, because an attachment sits in the recipient's mailbox and does not depend on your account continuing to exist.

What not to do

Renaming a .pdf to .txt to sneak past a filter does not change the size and will be caught, and putting the file in a password-protected archive to evade a scanner is worse: many corporate gateways quarantine encrypted archives precisely because malware uses that trick, so your file is now both large and suspicious. Splitting a file into multi-part archives asks the recipient to reassemble it and fails if any part is stripped. And sending the same 20 MB file to fifteen people multiplies it by fifteen on the server, which is how a mailbox quota disappears in an afternoon. None of these are clever; they are the reasons IT departments distrust attachments.

Choose the smaller file at the source

Much of this is avoidable upstream. Exporting from a design or office application, look for a screen or web preset rather than press quality — the difference is often a factor of five, and the press preset exists for a printing house, not for an inbox. Scanning your own documents, use 200 or 300 dpi in greyscale rather than 600 dpi in colour. Building a presentation, be aware that a single pasted phone photograph can carry twelve megapixels into a slide that displays it two inches wide. A file that was never bloated does not need rescuing, and the settings that prevent it cost nothing at the moment of export.

A decision in four steps

Check the size against the ten-megabyte planning number. If it is over and the document contains scans or images, compress it and re-check. If it is still over, ask whether the recipient needs all of it, and split if they do not. Only then reach for a link, and if you do, verify the permissions from outside your own account and say in the message when the link expires. Following that order, most oversized attachments are solved in the first two steps, and the recipient gets a file rather than a chore.