Email was never designed for big files. Gmail caps a message at 25 MB and Outlook.com at 20 MB, and because attachments are encoded as text on the way, a file grows by about a third in transit. In practice a PDF over 18 MB is a gamble.
What makes a PDF heavy
Almost always pictures. A page of typed text is a few kilobytes. A page scanned at 600 DPI in color can be 5 MB, and a 30-page scan is suddenly bigger than a short film. Embedded fonts add a little. Hidden leftovers from editing add a little more.
So before you compress, look at what’s inside. A report exported from Word with two charts has little to give. A scanned contract has plenty.
1. Compress the images inside
Compress PDF lowers the resolution and quality of the pictures and leaves real text alone. Start with the lightest strength and open the result. For reading on screen, 150 DPI is plenty. Go lower only if nobody needs to zoom in on small print.
2. Drop the color
A color scan of a black-and-white form stores three channels of nothing. Grayscale PDF turns it grey, or pure black and white, and scans often lose more than half their size. Don’t do this to anything where a red stamp or a blue signature matters.
3. Send fewer pages
If the person only needs pages 4 to 9, send pages 4 to 9. Split PDF pulls out a range, or cuts a long file into parts you can send in separate emails.
4. Send a link
When the file has to stay at full quality, like artwork for a print shop, don’t fight the limit. Upload it to your cloud drive and email the link. Gmail does this on its own for anything over 25 MB.
Check before you send
Open the smaller file and zoom to 100%. Read the smallest text on the busiest page: a table, a footnote, a stamp. If you can read it comfortably, so can they. Keep the original too, since compression can’t be undone.
One thing compression won’t remove is the author name and software details stored in the file. That’s a separate job for Remove PDF Metadata.
Common questions
Why did my PDF hardly shrink?
It probably holds text and vector drawings, which are already compact. Only pictures give big savings.
Does compressing a PDF affect its legal validity?
It can break a certificate-based digital signature, because the file changes. Compress first, then sign.
Can I search the text afterwards?
Yes, if the text was real text before. A scan is a picture of text and stays one unless you run OCR on it.