How to reduce a PDF's file size — and how to tell when it simply won't shrink

Nearly every guide on this gives you the same advice: drag your PDF into a compressor. What none of them mention is that the result depends almost entirely on what kind of PDF you have. Feed in a scan and you'll get 80% off. Feed in a contract and you'll get 5% off, still be over the limit, and have wasted a round trip. So diagnose first.

Step 1: Work out what kind of PDF you have

Open it and try to select a paragraph with your cursor. That one gesture tells you most of what you need to know.

The rule underneath all four: compression shrinks pixels, not text. A PDF stores text as instructions — “draw this character, in this font, here” — which is already about as compact as data gets. Images are stored as actual pixel data, which is where all the weight and all the opportunity live.

Step 2: The fix for your kind

If it's a scan (the big win)

Scanners default to settings meant for archiving artwork, not for emailing a signed form: 300 or 600 DPI, in colour. A ten-page contract scanned that way can easily be 40 MB. The same pages at 150 DPI greyscale land nearer 2 MB and look identical on screen.

1. If you can re-scan, do that instead of compressing. Set 150–200 DPI and greyscale. This beats any compressor, because you never create the bloat in the first place.
2. If re-scanning isn't an option, run it through a compressor and expect 70–90% off. This is the one case where the standard advice genuinely delivers.
3. Run OCR while you're there if your tool offers it. It won't shrink the file much, but it makes the text searchable and selectable, which is usually why you wanted a PDF rather than photos.

If it's image-heavy

Photos exported at print resolution are the culprit. A compressor will downsample them to something screen-appropriate and typically take 40–70% off. Check the result before sending: this is the one category where compression can visibly degrade the thing you're sending, and a portfolio with smeared photographs is worse than a large one.

If it's font-heavy

Design tools often embed complete font files — every glyph, including the thousands you never used. Subsetting the fonts (embedding only the characters actually present) is the fix, and most “optimise” or “export for web” options do it automatically. Re-exporting from the original design file usually beats post-processing the PDF.

If it's mostly text

Stop trying to compress it. There is no meaningful amount of air in a text PDF, and every hour spent on compressors is an hour you're not solving the real problem. If a text-only PDF is genuinely too big for where you're sending it, either the destination's limit is unusually tight or the document has something hidden in it — a full-page background image, or a logo embedded at print resolution on every page.

What to stop doing

• Zipping it. PDFs compress internally already and the images inside are usually JPEGs. You'll save two or three percent and cost the recipient a step.
• Running it through three compressors in a row. The second one has nothing left to take. That's what “this file is already compressed” means.
• Printing to PDF and hoping. It sometimes helps by flattening accumulated cruft, but it can also silently rasterise your text — turning a crisp, searchable document into a picture of one, which is both larger and worse.

The question worth asking first

Almost everyone who searches for this is trying to squeeze a file under an email attachment limit. That limit is lower than you think — Base64 encoding inflates every attachment by about a third, so a 25 MB ceiling is really an 18 MB file ceiling. Compressing a 40 MB text-heavy PDF down to that is not happening.

The alternative is to stop making the file fit the pipe. Upload the PDF and send a link instead: no size negotiation, no quality loss, the recipient opens it in their browser with nothing to download, and you can see when it was opened. The 40 MB scan goes as-is, at full quality, and the email carries a URL.

Quick answers

Related guides

• How to send a large PDF that's too big for email — all six methods, once you've decided compression isn't it.
• Email attachment size limits in 2026 — the real ceiling you're aiming at.
• How to turn a PDF into a link