How to Compress a PDF to Reduce File Size for Email

When a PDF is too heavy to attach to an email, you can compress it to reduce its size. This guide shows how to compress a PDF for free with Compress PDF, what each level changes in your file, and what to try when the size barely moves. There is no account to create and no watermark on the result.

What makes a PDF heavy

Most of the weight of a large PDF comes from images: photos, scanned pages and screenshots stored at high resolution. Embedded fonts add some weight, and so does the internal structure of the file.

Text itself is light. A document made only of text is usually small already, so compression has little to remove from it.

How to compress a PDF, step by step

1. 1 Open Compress PDF and drop your file on the page, or choose it from your device. The tool takes one PDF at a time and shows its name, page count and size.
2. 2 Under Compression level, choose Low, Medium or High. Medium is selected by default.
3. 3 Click Compress and download. The file is sent over HTTPS to the ProPDF server on AWS in the Paris region (eu-west-3), compressed there, then deleted as soon as the task ends.
4. 4 If the file is smaller, compare the Original and Compressed sizes and click Download. The new file has _compressed added to its name. If the chosen level does not reduce the size, no download is offered.

What the three levels change

Each level works on the images inside the PDF. An image drawn on the page at a higher resolution than the level's threshold is scaled down to the level's target, and each image is saved again as JPEG at the level's quality. The new version only replaces the original when it is more than 12% smaller, and transparent images or images of fewer than 8,100 pixels are left as they are.

• • Low: images above 160 dpi are reduced to 150 dpi, JPEG quality 82. The gentlest level, for photos and scans you want to keep detailed.
• • Medium: images above 140 dpi are reduced to 120 dpi, JPEG quality 68. A middle ground for documents that are read on screen.
• • High: images above 120 dpi are reduced to 96 dpi, JPEG quality 52. The strongest level, for files where size matters more than image detail.

What compression leaves alone

Text and vector drawings are not touched, so real text stays sharp and selectable. Text that is part of an image, such as a scanned page or a screenshot, is compressed with that image, so check that it is still readable at the High level.

Transparent images are left as they are, and so are very small ones, with fewer pixels than a 90x90 square. The result is never larger than your original.

Other changes to the file

On the server, the tool keeps only the characters actually used from each embedded font when it can, and it removes XMP metadata, while the document title and author stay. These changes, like those below, only reach you when the server returns a smaller file; the browser fallback described under Large files makes none of them.

It also resets interactive form fields to their default values and removes attached files, scripts and replies to comments. Keep the original of a filled-in form, and compress a copy.

When the file barely shrinks

A PDF made mostly of text, or one that was compressed before, has little left to remove. The reduction can then be small, or the tool shows This file is already optimized and offers no download, because the chosen level could not make the file any smaller.

Try a higher level if your images can lose some detail. You can also remove pages you do not need with Delete Pages, or keep only the pages that matter with Extract Pages; both run in your browser.

Compress PDF cannot open a PDF that asks for a password. Remove the password with Unlock first (you need to know it), compress the file, then add a password again with Protect if it must stay protected.

Large files

Server compression accepts files up to 4 MB. The page still sends a larger file first, and when the server refuses it, or fails for any other reason, the tool falls back to a lighter optimization in your browser, which the result screen points out. That fallback leaves images untouched and ignores the level you chose, and the size usually drops much less.

For a large scan, split it into smaller parts with Split PDF, compress each part, then join them again with Merge PDF.

Sending a compressed PDF by email

Email services set their own attachment limits, and they vary from one provider to another. Check the limit of your service before you pick a level, and keep in mind that the recipient's service has its own.

Before you send, open the compressed file and check that scans and photos are still readable. If it is still too large for one message, send it in several parts made with Split PDF.

For a confidential document, the panel that appears after the download can pass the file straight to Protect, which also runs on the ProPDF server in Paris. It adds AES-256 encryption, so the password is required to open the file: send that password through another channel than the email.