Compress PDF

How do I compress a PDF to make it smaller?

1. Drop one or more PDFs onto the drop zone, or click it to choose them.
2. Under Compression, pick Lossless (tidy structure only), Balanced (150 dpi images) or Strong (96 dpi images, smallest).
3. Click Compress PDF and watch the progress bar; big scans take a few seconds per page.
4. Compare the new size shown against the original.
5. Download the compressed PDF, or use Download all as ZIP if you compressed several files at once.

The three modes work in different ways, and choosing the right one matters. Lossless re-saves the document with pdf-lib using object streams and a cleaner internal structure, dropping unused objects and duplicated data. Text stays selectable and searchable and nothing visible changes, but the saving is usually small, typically 5 to 20% and sometimes almost nothing on a file that was already well built.

Balanced and Strong take a different approach. Each page is rendered with PDF.js to a bitmap, saved as a JPEG and placed on a fresh page of the same size. Balanced renders at 150 dpi, which looks clean on screen and prints acceptably; Strong renders at 96 dpi, which is fine for reading on a screen but visibly soft in print. On scanned documents and photo-heavy brochures these modes routinely cut the size by 50 to 90%, so a 30 MB scan can come out at 3 to 5 MB. The trade-off is real: after Balanced or Strong the PDF is a set of pictures, so you cannot select, search or copy the text, and links and form fields are gone.

A sensible rule is to try Lossless first on anything you typed yourself, such as a Word export or an invoice, and reach for Balanced when the file is a scan or must get under an email limit (Gmail 25 MB, Outlook.com 20 MB, most corporate mail 10 to 25 MB). Use Strong only when Balanced is still too big and the document just needs to be readable. You can compress several PDFs in one go and get them back as a ZIP.

There is no file size limit apart from your device's memory, no signup and no watermark. Rendering a 200-page scan at 150 dpi is heavy work for a browser and can take a minute on a phone, so be patient rather than reloading. Everything runs on your device with open-source libraries; the PDF is never uploaded, and nothing is stored.

How it works: this tool uses pdf-lib and PDF.js (Mozilla), open-source libraries loaded into your browser. No server is involved, so Kilnfile never sees your file. Last reviewed 20 September 2026.

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