Aim for under 18MB for personal Gmail and similar, and under about 7MB if the recipient's company runs its own Exchange server on the default 10MB limit. The stated limits are higher than the working ones because attachments are encoded for transit and grow by about 37 percent.
Then pick the technique that matches the file: remove blank pages from scans, re-export from the source for Office documents, compress images for everything else.
The attachment bounced, or the send button greyed out, and now the file has to get smaller. What most people do next is run it through a compressor, look at the result, and run it through again when the number is still too high.
That works for some files and does nothing for others, and the difference is predictable. It is worth thirty seconds of diagnosis before you start.
The number you are aiming at
The limit that stops an email is not always the number the provider advertises. Attachments cannot be sent as raw binary, so they are encoded into a text-safe format for transit, and that encoding inflates them by roughly a third. Gmail counts the file before encoding, but many receiving servers, Exchange among them, count the encoded message, so the file that reaches them has to be about a quarter smaller than their cap.
Google's sending limit, counted before encoding. The recipient's server may count the encoded size instead.
Base64 expansion plus MIME line breaks. Fixed, unavoidable, applies everywhere.
Raw file size that clears personal accounts reliably. Use about 7MB for a corporate server on a 10MB limit.
Work out what kind of file you have
Two questions settle which technique will help. Is the document scanned or born-digital? And does its size look proportionate to its page count?
- Scanned, very largeBlanks and downsampling
- From Word or Docs, largeRe-export from the source
- Few pages, enormousOne oversized image inside
- Many pages, text onlyAlready small, split instead
- Has photographsCompress, check quality after
- Already compressed onceGo back to the original
Five techniques, most effective first
Remove pages that carry no information
On a duplex-scanned document this is frequently the biggest single saving available, and it is the only one that costs nothing at all in quality. Half the pages are scans of blank paper, and each still carries an image of its own. Removing them halves the page count before any compression happens. Finding and removing blank pages covers the dust, shadows and show-through that make a blank scan look occupied.
Re-export from the original source
PDFs generated from Word, Pages or Google Docs vary enormously in size depending on the export settings used. A document exported with print-quality image embedding can be several times larger than the same document exported for screen viewing. If you still have the source file, re-exporting properly is faster than compressing and produces a cleaner result, because the images are downsampled once from the originals rather than re-encoded from already compressed copies.
Compress the images
The general purpose answer, and the right one when the document contains photographic content. Downsampling resolution to match the destination does most of the work, and re-encoding at lower quality does the rest. Compress PDF for Email does this in the browser and aims for a size Gmail will accept.
Split the document
Two 9MB emails reliably beat one 18MB email that bounces. Legal teams often deliver disclosure bundles as numbered parts for this reason. It is also the only technique that does not degrade anything.
Send a link instead
The fallback when the file cannot shrink and cannot sensibly be split. It works, and it changes the recipient's experience: a link rather than an attachment, sometimes a sign-in prompt, and a file that breaks if you ever move it.
Keeping it readable
There is a floor below which a document stops being useful, and hitting an arbitrary size target is not worth crossing it.
Smaller and still usable
- Text stays crisp at 100 percent zoom
- Resolution matched to how it will be read
- One pass, from the original file
- Checked on the page with the smallest text
- Original kept in case you need to retry
Smaller and no longer useful
- Grey halos around small text
- Photographs turned blocky and posterised
- Multiple passes stacking artifacts
- Checked at fit-to-window, where damage hides
- Original overwritten, no way back
Stopping the problem recurring
Most oversized attachments are made rather than found, and a few defaults prevent almost all of them.
Scan in greyscale unless colour carries meaning, since it stores one value per pixel instead of three. Scan at 300 DPI rather than 600: six hundred is an archival setting, and for a document that will be read rather than reproduced it is four times the pixels for no visible benefit.
Turn duplex off for single-sided originals, which halves the page count in one setting. And export for screen rather than press, because the press-quality preset in Office and design tools is wasted weight unless the file is going to a print shop.
Get the file under the limit
Compress in your browser and check the result before you send it. Nothing is uploaded.
Common questions
Getting under a specific number.
01What size should I aim for?
Work from the recipient's limit, not yours, and count every attachment together. The cap applies to the whole message, so two 10MB files in one email are judged as a single message of about 27MB once encoded. Two separate emails would each pass.
02Why did my email fail when the file is under the stated limit?
Because the server that refused it probably counts the message after encoding, which is about 37 percent larger than the file, and the message also carries its text and any other attachments. Gmail checks the file before encoding, but many receiving servers, Exchange among them, check the encoded message. The calculator above shows what a given file size becomes on the wire.
03What is the fastest way to shrink a scanned document?
Remove the blank pages first. Duplex scanning of single-sided paper doubles the page count with pictures of empty paper, and deleting them costs nothing in quality. Compress afterwards, so the compressor does not spend effort on pages you were going to delete.
04Is it better to compress or to split?
Split when the content needs its resolution, such as drawings or photographs, or when compression has already reached the point where text is suffering. Two smaller emails always arrive. One oversized email does not.
05Why is my PDF huge when it is only three pages?
Almost certainly one oversized image. A photograph placed at full camera resolution and scaled down on the page keeps every one of its pixels in the file, even though only a fraction of them are visible. Compression fixes this well because there is so much redundancy to remove.
06Does compressing twice help?
No. The second pass works on the artifacts left by the first and treats them as detail worth preserving, so you lose noticeable quality for very little size. If the first result is still too large, return to the original and compress harder in a single pass.