Key takeaways
- PDF compression works by re-rendering each page's images at lower resolution, which routinely cuts scans and image-heavy files by more than half.
- A compressed PDF behaves like a scan: text stays readable and printable but can no longer be selected or copied, so keep the original.
- Start with the balanced level, and if a stronger setting is needed re-run the original file rather than the compressed copy so quality loss does not compound.
- Text-only PDFs are already compact and will not usefully shrink; oversized scans are better rescanned at 300 DPI or shared as a cloud link.
When an email or upload portal rejects your PDF for size, compression is the standard fix. Most of a PDF's bulk lives in its images, so shrinking those is where the savings come from — which is why scans and image-heavy files compress dramatically while text-only files barely budge.
What compression actually changes
A PDF's size lives in its images. Our Compress PDF tool re-renders each page as an optimized image at a resolution set by the level you choose, then rebuilds the document — entirely in your browser, so the file never leaves your device. On scans and image-heavy PDFs this routinely cuts the size by more than half. The trade-off is that the compressed copy behaves like a scan: text stays readable and printable, but it can no longer be selected or copied, so keep your original if you need to edit it later. A text-only PDF is already compact and will not usefully shrink; the tool tells you so rather than producing a larger file.
How to compress a PDF
- Open the free Compress PDF tool.
- Upload your PDF. The tool shows its current size.
- Choose a compression level, then compress.
- Check the before-and-after size, download the smaller file, and send it on its way.
Choosing the right level
High quality applies light compression, best when the document will be printed or examined closely. Balanced is the everyday choice, noticeably smaller with little visible difference on screen for most documents. Small file compresses aggressively for the times you must fit a hard limit, trading image sharpness for maximum size reduction. When unsure, start with balanced — and if you need to go stronger, re-run the original rather than the compressed copy, so the quality loss does not compound.
When compression is not the answer
If a compressed file is still too large, the images inside it may simply be enormous, such as pages scanned at very high resolution. Rescanning at 300 DPI, or rebuilding the PDF from resized images, often beats squeezing the oversized original. And when a file must stay at full quality — or its text must stay selectable — share it as a cloud link instead of an attachment.
The bottom line
Pick balanced, check the before-and-after size, and go stronger only if the limit demands it. Compression spends its savings where the size actually lives — the images — so scans shrink dramatically, while already-compact text files are best sent as they are.