How to reduce PDF file size for email

Step by step

1. Open Compress PDF. Go to the Compress PDF tool. The page loads the compressor into your browser tab, and the file never leaves your Mac or PC.
2. Drop in the PDF. Drag the file onto the drop zone or click it to browse. Add several PDFs at once if you have more than one to send.
3. Choose a compression level. Under Compression pick Lossless (tidy structure only), Balanced (150 dpi images) or Strong (96 dpi images, smallest). Balanced is the right first choice for a scanned document.
4. Click Compress PDF. The tool shows the original and new sizes side by side, so you know whether you are under the limit before you download anything.
5. Check the result. Open the compressed file and zoom in on a page of small print. If it looks soft, go back and try Balanced instead of Strong; if the text must stay selectable, use Lossless.
6. Download and attach. Click Download, or Download all as ZIP when you compressed several files, then attach the compressed copy to your email.

What are the email attachment limits?

Every mail provider caps the size of a single message, and the cap applies to the whole message rather than each attachment. The figures below are the ones that matter for UK users. Note that a 24 MB PDF can still bounce from Gmail, because the attachment is encoded for transit and grows by roughly a third on the way.

Corporate mail is the usual surprise. Many company and NHS mail servers reject anything over 10 MB, and the bounce often arrives hours later, so if you are sending to a work address aim well under that figure.

How much can compression save?

It depends almost entirely on what is inside the PDF. A document exported from Word or Google Docs is mostly text and fonts, and there is little to squeeze: Lossless typically saves 5 to 20 percent by tidying the file structure and removing duplicate objects. A scanned document is a different matter, because each page is a large photograph. Balanced re-renders those pages as JPEG at 150 dpi and Strong at 96 dpi, which is where the 50 to 90 percent savings come from.

Which setting should I choose?

Start with Lossless if the PDF came from a word processor, because it keeps the text selectable and searchable and the saving is often enough on its own. If the file is still over the limit, or it is a scan, move to Balanced. Strong is for the stubborn cases: a 40-page scanned contract that must go to a 10 MB corporate mailbox.

Notice the third row in the table above. A text-only PDF gets bigger under Balanced and Strong, because rendering a crisp text page as a JPEG costs more bytes than storing the text itself. If the compressed size goes up, use Lossless and look for another route.

Why does the text stop being selectable?

Balanced and Strong work by rasterising each page: the page is drawn as a picture and saved as a JPEG, and the original text layer is discarded. That is the trade for the big saving. The recipient can still read and print the document normally, but they cannot search it, copy from it or have a screen reader speak it. For a signed form or a scan that was never searchable in the first place this costs nothing; for a report someone will quote from, prefer Lossless.

What if the PDF is still too big?

Zipping a PDF rarely helps, because the images inside are already compressed. Better options are to split the document with Split PDF and send it as two or three messages, to delete appendix pages that the recipient does not need with Delete PDF Pages, or to share a link from iCloud, OneDrive or Google Drive instead of an attachment. Links are also kinder to the recipient's mailbox quota.

Is it safe to compress a confidential PDF in the browser?

Yes. Kilnfile runs the compressor as code inside your browser tab, so the contract, payslip or medical letter is read from your disk, processed in memory and written back to your Downloads folder. There is no upload step and no server copy to be leaked or retained. If you want to confirm this, open your browser's developer tools on the Network tab while compressing: you will see the tool's code load once and no outbound file transfer.

Frequently asked questions