Aim for under 18MB for personal Gmail and similar, and under 10MB if the recipient is on a corporate address. 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.
What number you are actually aiming at
Email attachment limits are not the numbers the providers advertise. 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. The advertised cap applies to the encoded size, not to the file on your disk.
Applies to the encoded attachment on the wire, not the file in your folder.
Base64 expansion plus MIME headers. Fixed, unavoidable, applies everywhere.
Raw file size that clears personal accounts reliably. Use 10MB for corporate.
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 photographs of blank paper at 20 to 80KB each. Removing them can halve the file before any compression happens.
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 genuine photographic content. Downsampling resolution to match the destination does most of the work, and re-encoding at lower quality does the rest. This is where a compressor earns its place.
Split the document
Two 9MB emails reliably beat one 18MB email that bounces. This is standard practice in legal work, where disclosure bundles are routinely delivered as numbered parts. It is also the only technique that does not degrade anything.
Send a link instead
The fallback when the file genuinely 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, greyscale is roughly a third the size for an identical looking document.
Scan at 300 DPI, not 600. Six hundred is an archival setting. For anything that will be read rather than reproduced, it is four times the pixels for no visible benefit.
Turn duplex off for single-sided originals. It halves the page count and the file size in one setting.
Export for screen, not press. Unless a printer is genuinely involved, the press-quality preset in Office and design tools is wasted weight.
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?
Under 18MB for personal accounts such as Gmail, and under 10MB if the recipient is on a corporate address. Both are below the advertised caps because attachments grow by about 37 percent during encoding, and the limit applies to the encoded size.
02Why did my email fail when the file is under the stated limit?
Because the limit applies to the attachment after encoding, not to the file on your disk. A 22MB PDF becomes roughly 30MB in transit, which is over a 25MB cap even though the file itself is not.
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 photographs of empty paper, and deleting them often halves the file with no quality cost at all. Compress afterwards.
04Is it better to compress or to split?
Split when the content genuinely 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.