How PDF compression actually works (lossless vs. strong)

PDF compression means reducing the file size of a PDF. There are two fundamentally different strategies, and understanding them explains why some files shrink dramatically while others barely budge.

What makes a PDF big

A PDF is a container holding text instructions, embedded fonts, and usually images. In a scanned contract, page images make up 95%+ of the bytes. In a generated invoice, the file is mostly compact text operators, and there is simply less to squeeze.

Lossless optimization

Lossless mode rebuilds the PDF structure: unused objects are dropped, metadata is stripped, and the file is re-saved with object streams that pack data more efficiently. Quality is untouched and text stays selectable. But it can only remove redundancy; a lean, text-heavy PDF may shrink by just a few percent.

Strong mode (rasterization)

Strong mode renders each page to an image at a chosen DPI and rebuilds the PDF from compressed JPEGs. This is where the big wins live: a 30 MB scan can become 4 MB. The trade-off: text becomes part of the image. No selecting, no searching, no copying. For scans, that is usually irrelevant because the text was already an image.

Practical recommendations

For documents you'll print or archive: try lossless first, and keep strong mode at 150 DPI / 70% quality. For screen-only sharing, 96–120 DPI is usually plenty. Always keep your original file. Compression is a one-way door.