How to compress a PDF without ruining it
A PDF that is too big for an email or an upload form is one of the most common small annoyances at work. The fix is simple once you know what is inside the file.
What makes a PDF large
A PDF is a container. Its size usually comes from a few things:
- Images. Scans and photos dominate file size. A scanned page is often a single large image, and 20 scanned pages can easily be tens of megabytes.
- Embedded fonts. Each font used in the document may be embedded, sometimes in full.
- Structure and metadata. Page trees, annotations, form data, thumbnails and edit history that some editors leave behind.
Plain text is tiny. A 200-page book of text can be smaller than one photo.
Two very different kinds of compression
Lossless: tidy the container
Lossless optimisation rewrites the file structure more efficiently (for example by packing objects into compressed streams) without altering how any page looks. Text stays selectable and searchable. The catch: if the file is mostly images, you will save little, because the images themselves are untouched.
Image-based: reduce the pixels
For scans, the big win is re-rendering each page at a lower resolution and saving it as a JPEG. The result can be a fraction of the original size. The trade-offs:
- Text becomes part of the picture, so it can no longer be selected, searched or copied.
- Quality depends on resolution. Around 150 dpi looks fine on screen and acceptable in print; 100 dpi and below starts to look soft.
Which setting to choose
| Your file | Try |
|---|---|
| Mostly text, created from Word or a web page | Lossless. It is probably already small. |
| Scanned documents, forms, signed pages | Balanced (150 dpi) |
| Need to fit a strict limit such as 1 MB | Strong, then check legibility |
| Photos or slides with detailed images | Balanced; zoom in to check |
If a setting produces a file that is not smaller, the Compress PDF tool returns your original instead of making things worse.
Tips that save more than any setting
- Compress last. Merge, reorder and add page numbers first, then compress once. Repeated lossy compression degrades quality each time.
- Drop pages you do not need with Remove Pages or Remove Blank Pages. Scanners often add blank back sides.
- Keep the original. Image-based compression cannot be undone.
- Scan smarter. If you control the scan, 150–200 dpi grayscale is plenty for most documents.
Is it safe to compress sensitive documents online?
It depends where the compression happens. Many services upload your file to a server. Tools that run in your browser, like this one, process the file on your own device and never send it anywhere. See how to verify that claim.