Why "nothing uploaded" matters
Uploading a PDF to a compression website puts a copy on someone else's server, under their rules on how long it's kept and where. That may be fine for a menu. It's less fine for a tenancy agreement, a medical letter, staff payslips or a client's tender drawings. If the PDF holds other people's personal data, handing it to a third-party service is a disclosure you may need to account for under UK GDPR.
A tool that works on your own computer avoids the question. One quick test for any tool: a website that uploads files can't process them while you are offline.
What makes a PDF big
| What's inside | Why it's big | What compression does |
|---|---|---|
| Phone photos | A 12 MP photo placed on a page is often stored at around 600 dpi | Reduces it to about 150 dpi, which still prints sharply at the size shown |
| Colour scans | Scanned at 300 dpi or more, every page a full picture | Re-encodes, and lowers the resolution on the smaller settings |
| Black-and-white scans saved as grey | Grey pictures where only black and white are needed | Stores them as true black and white, often a small fraction of the size |
| The same picture on every page | A logo or title block stored again on each page | Stores it once |
| Hidden extras | Thumbnails, metadata and private data left by drawing programs | Removes them |
| Text and line work | Usually already small | Left as it is, so it stays sharp and searchable |
Step by step in your browser
- Open Compress PDF in your browser. There's nothing to install and no sign-up.
- Add your PDFs - up to three at a time in the free edition.
- Look at the Check tab. It shows where the size comes from (images, fonts, line work, extras) and the heaviest pages, so you know what to expect before you start.
- Pick a setting: Best, Balanced or Smaller (see the table below).
- Compress, then use the Compare tab to slide between before and after and zoom into small print.
- Save. The copy ends
_compressed.pdf, your original isn't touched, and a file is never made bigger.
| Setting | Photos | Scans | Black and white | Use it for |
|---|---|---|---|---|
| Best | 225 dpi | 300 dpi | 600 dpi | Documents that will be printed |
| Balanced | 150 dpi | 200 dpi | 300 dpi | Most email and sharing |
| Smaller | 110 dpi | 150 dpi | 300 dpi | Getting under an attachment limit |
What to expect: real results from our test files
| File | Before | After (Balanced) |
|---|---|---|
| 6 A1 drawings with a large title block on each | 28.3 MB | 210 KB |
| Site photo report, 8 phone photos | 27.3 MB | 449 KB |
| Delivery notes: 4 colour and 2 black-and-white scans | 17.1 MB | 4.6 MB (541 KB on Smaller) |
| A letter exported from a word processor | 57 KB | 52 KB |
Every output had the same page count and the same text as the original. Your own files will differ: the Check tab is the honest guide to what's possible.
Getting under an email attachment limit
Many email services cap attachments at 20 to 25 MB, and some company mail servers at 10 MB. Remember that attachments grow by about a third when they are emailed, so aim comfortably below the limit.
- Try Smaller first; check small print in the Compare view.
- If it still won't fit, split it into parts. Compress PDF Pro (Windows) does this automatically at page boundaries, or the free PDF Split & Merge app lets you pick the pages by hand.
- For very large drawing sets, a shared link from your file storage may be better than an attachment.
What compression won't do
- It won't open password-protected PDFs. Save an unprotected copy from the program that made the file first.
- It won't make a text-only document much smaller, because there's little to remove.
- It doesn't turn pages into pictures, so drawings keep their vector lines and text stays searchable. If you want the opposite - pages that can't be copied - that's a different job.