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Compressing a PDF to fit an email limit
Most of a large PDF is images, so compression works by re-encoding them at a lower resolution. In our tests a 1.15 MB scan dropped to 119 KB, a 90% reduction. A text-only document usually cannot shrink at all, because there is nothing to re-encode.
What makes a PDF large
A PDF is a container. Text, fonts, links and page geometry are small — a 40-page text document is often under a megabyte. Photographs and scans are not: a single A4 page scanned at 300 DPI can weigh several megabytes before any compression.
That is why two files with the same page count can differ by a factor of fifty. The one that is heavy is almost always the one with pictures or scans in it.
What compression actually changes
Image compression decodes the picture inside the PDF, scales it to a sensible resolution, and re-encodes it as a smaller JPEG. A page that will be read on a screen does not need 300 DPI; 150 is usually indistinguishable and a quarter of the data.
Text is left alone. It stays vector, which means it remains selectable and searchable, and it does not blur the way a rasterised page would.
How much smaller it gets
Measured on our own fixtures, with the Recommended level:
- 1.15 MB scan at 200 DPI → 119 KB (90% smaller).
- 579 KB scan at 150 DPI → 233 KB (60% smaller).
- 17 KB invoice with one photograph → 10 KB (41% smaller).
- 2 KB text-only document → unchanged, reported as already optimised.
When it will not work
If a PDF contains no large images, there is nothing to shrink. Some tools respond by rewriting the file anyway and handing back something slightly larger; a good one tells you the file is already efficient and returns the original.
Images stored in formats other than JPEG also resist compression in most browser tools. A scan saved as a PNG may barely move. Converting it first, or using a tool that re-encodes other formats, is the way around it.
Choosing a level
Use the middle setting for anything that will be read on a screen or printed at A4. Drop to the strongest setting when the limit is tight and the page is mostly photographs; keep the lightest setting when the document contains fine print or diagrams that must stay legible.
If the result is still too large for the limit, splitting the document and sending two emails beats compressing until the text is unreadable.
Questions
What is the usual email attachment limit?
Gmail and most consumer providers cap a single message at 25 MB, and many corporate systems cap it at 10 MB. Because attachments are base64-encoded, a 10 MB file becomes about 13 MB on the wire, so aim for roughly 8 MB to be safe.
Does compressing a PDF make the text blurry?
No, if the tool re-encodes images only. Text stays vector and stays sharp. Blurring happens when a tool rasterises whole pages, which is a different and lossier operation.
Why did my PDF not get smaller?
It probably contains no large images. Text, fonts and links take very little space, so there is nothing to re-encode. A tool that reports "already optimised" is giving you the correct answer.
Can I compress a password-protected PDF?
Not directly. Remove the password first, compress the copy, then protect it again if it still needs to be locked.
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