Lossless and Lossy Compression
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Definition
Section titled “Definition”Lossless compression rebuilds a file so that its output is byte-substitutable for its input in everything the reader experiences: every word, every path, every image unchanged. Lossy compression discards information the human eye barely notices — detail in a photograph, high frequencies in audio — to reach sizes lossless methods cannot. Lossless is the right default for documents; lossy is the lever on image-heavy files, and it is a decision, not a setting.
Example
Section titled “Example”A text-heavy report compressed by rebuilding its object streams: the file is smaller, and comparing the pages before and after shows no difference at all, because nothing in the rendered output changed. The same file run through an aggressive lossy pass on its images shows smaller size and visible artifacts — blocky text in screenshots, blurred gradients in charts. One is a free win; the other is a trade someone has to choose.
In the PDFLean tools
Section titled “In the PDFLean tools”- compress a PDF — the lossless structural operation
- compress an image — the lossy lever, applied to the source images
- compress PDF to 200KB — the target-size tool, whose trade is that the text layer becomes images
Related reading
Section titled “Related reading”- Raster and Vector — why lossy compression only affects the raster parts of a page
- Object Streams — what the lossless pass actually rebuilds
- Compress PDFs — the family hub, organized around this distinction