About this tool
Compress PDF is a free, in-browser tool that reduces PDF file size using lossless optimization: it rebuilds the file with object streams, compresses uncompressed streams and re-flates existing ones at the highest compression level. Because it runs on a WebAssembly build of qpdf directly in your browser, your file never leaves your device — nothing is uploaded to any server.
Lossless means quality is completely untouched: text stays sharp, images keep every pixel and the document remains identical on screen and in print. The savings come purely from restructuring the file, so results depend on how wastefully the PDF was written. Files exported from office suites or generated with old tools often shrink noticeably; already-optimized files may not shrink at all, and the tool tells you honestly when yours is already as small as this method can make it.
After compressing you see the before and after sizes and the percentage saved, and the smaller file downloads automatically. Scanned or photo-heavy PDFs are dominated by image data that lossless restructuring cannot reduce — deliberately, because recompressing images would degrade quality.