Your PDF is too big to email

The limits you are actually hitting

Worth knowing before you start, because the target decides how hard you need to work. Gmail and Outlook.com both stop at 25 MB. Microsoft 365 mailboxes are commonly capped at 25 MB too, though an administrator can set it lower and frequently has. Many corporate mail gateways enforce 10 MB regardless of what the mailbox allows, and some government and legal filing portals are stricter still — 5 MB is a common figure, and a few court systems specify less.

There is a second, invisible limit. Mail attachments are encoded as text in transit, which inflates them by about a third — so a 20 MB file can arrive as roughly 27 MB of message and be rejected by a 25 MB limit it appeared to satisfy. If a file just under the limit bounces, that is why. Aim for about three quarters of the stated cap and the problem stops happening.

Work out which kind of large file you have

Open the PDF and scroll through it. You are looking for one of three things, and the answer decides everything that follows.

Scans or photographs on most pages. Each page is a picture, and pictures are where the megabytes live. This is the common case and the one that compresses best — expect to lose half to four fifths of the size with no visible difference on screen.

Mostly text, but a great many pages. A 400-page text document is large because it is long, not because anything in it is wasteful. Compression has almost nothing to work on here, and this is the case where people try compressing repeatedly and conclude the tool is broken.

A few pages that are enormous. Usually a CAD drawing, a map, a poster or a page carrying one very high-resolution photograph. One page can account for most of the file, and finding it is often the whole job.

If it is scans or photographs: compress it

This is the fix that works, and it works because the images in a scanned PDF are almost always stored at a resolution far beyond what anyone will look at. Compressing the PDF re-encodes those images at a sensible ceiling and leaves text and vector artwork untouched, so the words stay perfectly sharp while the pictures give up detail nobody can see on a screen.

Expect 50 to 80 per cent off a scanned document at the recommended setting. A 40 MB scanned contract landing between 8 and 20 MB is a normal result. If you need it smaller still, the stronger settings trade visible image quality for size and say so — and the strongest setting turns each page into a single picture, which makes the file dramatically smaller and destroys the text layer, so the document can no longer be searched, selected or copied. Use it only when size is genuinely the only thing that matters.

One caveat for anything heading to a printer: compression converts images to screen colour. That is invisible on a monitor and matters on a press. See preparing a PDF for print.

If it is text: stop compressing and split it

Text in a PDF is already stored extremely efficiently — a page of prose is a few kilobytes — so there is no fat to remove and compression will return a few per cent at best. Running it repeatedly degrades the images that are there without helping the size, which is the worst of both outcomes.

The fix is to send less at a time. Splitting the PDF into sections that fit is usually the right answer for a long report, and it is frequently better for the recipient as well — nobody wants a 400-page attachment when the relevant part is chapter nine. Extracting the pages that matter is better still when you know which ones they are.

If a few pages are enormous: find them first

Split the document into single pages and look at the file sizes. The offending page is usually obvious — one page at 30 MB among fifty at 200 KB. Once you know which it is, you have options the whole-document approach does not give you: replace the image on that page with a smaller version, remove the page if it is a decorative cover, or send that page separately.

A scanned page saved as PNG is a specific and common version of this. PNG is lossless, which is right for screenshots and diagrams and badly wrong for a photograph of a document — the same page as JPEG is frequently a tenth the size at a difference nobody can see. See which image format to use.

When not to email it at all

Past about 20 MB, a link is better than an attachment and it is worth saying so plainly. Attachments are copied into every mailbox they touch, bounce without explanation at gateways you cannot see, and are the single most common reason a message silently fails to arrive. A shared link from whatever storage you already have avoids all three and lets you replace the file after sending it.

The exception is anything that has to be a matter of record — a filing, a signed contract, an invoice — where the attachment is the evidence that something was sent. There, compress and split as needed, and check the recipient's stated limit rather than assuming 25 MB.