Lossy vs. Lossless PDF Compression Explained

Compression algorithms generally fall into two categories: lossless, which shrinks a file without discarding any information, and lossy, which reduces file size by permanently discarding some detail in exchange for a smaller result. The two aren't interchangeable — which one applies depends on what's actually inside the file.

PDFPilot's Compress PDF tool uses lossy compression. Each page is rendered to a canvas and re-encoded as a JPEG image at a quality level tied to the setting you choose — Maximum Compression (scale 0.6, JPEG quality 0.5), Medium (0.8, 0.7), or High Quality (1.0, 0.92). JPEG itself is a lossy image format, so some visual detail is discarded at every setting; higher settings simply discard less.

This is a deliberate trade-off, not a limitation to work around. Scanned documents and photo-heavy pages are usually already raster images, so re-encoding them at a lower quality is an effective way to shrink them significantly. The cost is that the resulting PDF's pages are images — meaning text is no longer selectable, searchable, or copyable, regardless of which quality setting you pick.

A true lossless approach to PDF size reduction exists too — for example, removing unused embedded fonts or recompressing already-lossless image data more efficiently — but that typically saves far less space on documents that are already text- or vector-based, and none of it changes the trade-off for scanned or image-heavy PDFs. If keeping your text selectable matters more than achieving the smallest possible file, compression may not be the right tool for that particular document.

As with every tool on PDFPilot, this happens entirely in your browser. Your file is never uploaded to a server, at any quality setting.