How to reduce PDF file size: lossless vs. image compression
Published · 3 min read
Two PDFs with the same number of pages can differ in size by a factor of a hundred. Knowing why makes it easy to pick the right kind of compression, and to know when a file simply won't get much smaller.
What makes a PDF large
- Images. Scans and photos are almost always the biggest part of a large PDF. A page scanned at high resolution can weigh several megabytes on its own.
- Embedded fonts. Documents carry the fonts they use so they look the same everywhere. Usually small, but they add up in documents with many typefaces.
- Leftovers. Editing a PDF repeatedly can leave unused objects behind, and some older software writes data without compressing it at all.
Option 1: lossless compression on your device
Lossless compression removes unused objects and compresses data that was stored uncompressed. Text, images and layout are left exactly as they are, so there is no quality loss at all.
It helps most with files from older software or documents that have been edited many times. On a modern, already-optimised PDF the saving can be small, and that's normal. In Compress PDF, choose On this device. It runs in your browser and accepts files up to 500 MB.
Option 2: convert pages to images, still on your device
For scans, the local option can also re-render every page as a JPEG at the resolution and quality you choose. That can shrink a scanned document considerably without it ever leaving your computer.
The trade-off is real: text is no longer selectable or searchable, and links and form fields are removed. Use it for scans and pictures, not for documents someone needs to search or fill in.
Option 3: advanced compression on our server
Advanced compression uses Ghostscript and qpdf to downsample images while keeping text sharp and selectable. Choose Balanced, Smaller file or Maximum compression, or set a target size. The target is a best effort: we try a few quality levels and tell you whether it was reached.
Because this runs on our server, it's labelled before you start. The file travels over TLS and is deleted as soon as your browser has received the result, and in any case within about an hour. Files can be up to 100 MB.
Which one should you choose?
- Confidential or mostly text: start with lossless compression on your device.
- Scans or photo-heavy documents where text must stay selectable: advanced server compression usually saves the most.
- Scans where privacy matters more than selectable text: convert pages to images on your device.
- Already small and well optimised: some PDFs can't shrink much. If compression would make a file larger, you get the original back and we tell you so.
Other ways to make a file smaller
Sometimes the fastest way to a smaller file is to send less of it. Split PDF lets you keep only the pages that matter. If someone only needs to see a page, PDF to JPG turns it into an image. Both run in your browser.