How to compress a PDF so it fits in an email

You hit send and the message bounces back: attachment too large. Gmail caps attachments at 25 MB. Outlook is stingier at 20 MB, and plenty of company mail servers set their own limit lower than that. So your 38 MB scanned contract isn’t going anywhere until it loses some weight.

Good news: most oversized PDFs are oversized for one boring reason, and you can usually fix it in a few seconds without uploading anything.

Why a PDF gets so heavy

A PDF is really a container. Text and fonts barely weigh anything. The bulk almost always comes from images.

When you scan a document or export from a phone, each page is stored as a photo, often at 300 dpi or higher. That resolution is great for printing, but it’s wildly more than a screen needs. A single full-color scanned page at 300 dpi can run several megabytes on its own. Twenty pages and you’re well past any email limit.

Fonts add a little too. If a document embeds full font files instead of subsets, that’s extra baggage. But images are the thing. If your PDF is heavy, it’s almost certainly the pictures inside it.

Shrinking it in your browser

The files.co Compress PDF tool does the whole job on your own device. The file is read into memory, recompressed there, and saved back to your downloads. It never touches a server, so even a confidential contract stays on your machine. Load the page once and it works offline too.

You pick a compression level, and each one is a trade between file size and image quality:

• Extreme. Squeezes the file as hard as it can. Images get downsampled and compressed aggressively, so fine detail softens and photos can look a bit muddy. Use it when you need to clear a hard limit and the document is mostly text you just need to be readable.
• Recommended. The sensible middle. It drops resolution to something a screen actually uses and applies sane compression. Most documents lose half their size or more and still look clean on a monitor. Start here.
• Less. Light touch. It trims the obvious waste but keeps images close to the original. Pick this when the PDF has photos or diagrams where detail matters and you only need to shave off a bit.

The honest part: compression that throws away data can’t be undone. Text stays crisp because it isn’t an image, but downsampled pictures don’t come back. So keep your original. If the compressed version looks rough, run it again at a lighter level rather than trying to recover the squashed one.

A quick way to choose: try Recommended first, open the result, and zoom in on a page with images. If it looks fine, you’re done. If a limit forces your hand, drop to Extreme. If the images turned ugly and you’ve got room to spare, go back up to Less.

When you shouldn’t compress

Sometimes the file is already as small as it’s going to get, and compressing again just degrades quality for almost no gain.

If a PDF is mostly text, like an invoice exported straight from accounting software or a contract generated from a template, it’s probably already tiny. Running it through compression won’t help, because there were never heavy images to shrink. You might even make a vector-clean document look worse for the sake of a few kilobytes.

The same goes for a PDF someone already optimized. If you compress a file down to 4 MB, send it, and the other person compresses it again, the second pass costs quality and saves almost nothing. One good pass is enough.

Quick rule: if the file is already comfortably under your email limit, leave it alone. Compression is for the 30 MB monster, not the 2 MB one.

Check the size before you send

Right-click the file and look at its properties to see the size, or just glance at it after downloading. If it’s under 25 MB for Gmail or 20 MB for Outlook, you’re clear. If the recipient is on a stricter corporate server, aim lower, around 10 MB, to be safe.

And if you’re ever curious whether your file really stayed local, open DevTools (F12), go to the Network tab, and run a compression. You’ll see the page load up front and then nothing while the work happens. No upload, no copy on someone else’s server, just a smaller PDF on its way out.