How do PDF compressors reduce file size without losing quality?

Short answer
PDF compressors reduce file size through a combination of techniques: downsampling embedded images to a lower resolution, recompressing images using more efficient codecs (JPEG for photos, JBIG2 for black-and-white text), removing embedded fonts that are not used, stripping metadata and hidden layers, and applying lossless compression to the document structure itself. The amount of quality loss depends on the compression settings chosen - aggressive compression targets images, while lossless modes preserve full fidelity.

PDFs can grow to large sizes because they bundle everything the document needs to render identically on any device: embedded fonts (sometimes multiple weights), high-resolution images at their original DPI, vector graphics, colour profiles, document metadata, revision history, and sometimes embedded attachments. A PDF exported from InDesign or Illustrator with print-quality settings can easily reach 50-200MB, even if the visual output would look identical at screen resolution.

Image compression is where the biggest size savings come from. A PDF with high-resolution photos at 300 DPI for print can have its images downsampled to 150 DPI for screen use with no perceptible difference on a monitor. JPEG compression at 85% quality retains excellent visual fidelity while halving the image data. For scanned document pages that are all black text on white, JBIG2 compression is dramatically more efficient than storing each page as a JPEG image.

Font subsetting is another meaningful optimisation. When a PDF embeds a full font file (which can be 100-500KB per font), it includes glyphs for every character the font supports. If your document only uses basic Latin characters, subsetting embeds only the glyphs actually used, often reducing font data by 80-90%.

Lossless compression on the PDF structure itself uses Flate (ZIP-equivalent) encoding on object streams. This is always safe to apply as it is fully reversible.

The practical trade-off is straightforward: for documents intended for print, preserve high resolution. For documents shared by email, uploaded to websites, or emailed to clients, compress aggressively - 150 DPI, 80% JPEG quality, and font subsetting typically reduce a 20MB PDF to under 2MB with no visible quality difference on screen.

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