Compress PDF online, free

Compressing a PDF with Unido specifically means recompressing the images the document contains: photographs, scanned pages, or bitmap graphics. This tool doesn't rely on any external library: the engine that analyzes the PDF, decodes its images, and re-encodes them was written specifically for Unido, and it runs on Canvas 2D directly in your browser.

The engine walks through the resources of each PDF page looking for embedded images. Ones already in JPEG format are decoded with the browser's own image decoder and re-saved at a lower JPG quality, whichever you choose: "Recommended" or "Maximum compression". Images stored uncompressed (black-and-white, gray, or color bitmaps with no image compression filter) are decoded manually, pixel by pixel, and converted to JPEG for the first time — usually the biggest saving of all, since an uncompressed bitmap can weigh many times more than its JPG equivalent. If an image is very large in pixel dimensions, its resolution is also reduced before recompressing it.

For safety, this tool only replaces an image when the compressed result is genuinely smaller than the original — if an image is already heavily compressed and re-encoding it saves nothing, it's left as-is. And there are image types this tool doesn't touch at all, to avoid the risk of decoding them incorrectly: images in CCITT fax format, JPEG2000, indexed color palettes, Separation/DeviceN inks, CMYK color space, or with a transparency mask (SMask or Mask). All of those are copied unchanged into the final document.

This tool is upfront about what it doesn't do: it doesn't touch the text, fonts, or vector graphics of the PDF, so a document that's mostly text (a contract, an exported spreadsheet, an ebook) will barely change size, since there aren't large images to compress. It also doesn't do font subsetting or optimize page content streams. If, after analyzing your PDF, the result doesn't end up smaller than the original, Unido simply returns the original file unchanged, instead of handing you a "compressed PDF" that actually weighs the same or more.

When it's done, you'll see the original size, the final size, and the percentage saved — always with the real numbers for your file, whether it gains a lot, a little, or nothing from compression.