PDF Compression Explained: How It Works and When to Use It

A PDF's file size is almost never dominated by its text. Text and vector graphics (like drawn lines, shapes, or most fonts) are stored as compact instructions and take up very little space, even across hundreds of pages. The overwhelming majority of PDF file size comes from embedded raster images — photos, scanned pages, or screenshots — because each one stores a full grid of pixel data.

A scanned document is often the worst case: instead of storing selectable text, each page is a single high-resolution image, so a 20-page scanned contract can easily be larger than a 200-page text-based ebook.

PDF compression tools work primarily by re-encoding the embedded images at a lower quality or resolution, and by removing redundant internal data — duplicate font subsets, unused objects, uncompressed streams, and old revision history that some PDF editors leave behind. Downsampling an image (reducing its pixel dimensions) and increasing JPEG-style compression on it are the two levers that account for most of the size reduction in a typical file.

Text and vector content generally isn't touched, which is why compressing a text-heavy PDF (a report, an invoice, a contract) usually yields a smaller size reduction than compressing an image-heavy one (a scanned book, a photo portfolio, a slide deck full of screenshots) — there's simply less to compress.

Two 10MB PDFs can compress to very different sizes depending on what's inside them. A PDF full of already-compressed JPEG photos has little room left to shrink further without visible quality loss. A PDF containing uncompressed or lightly-compressed TIFF/BMP images, or a scan saved at unnecessarily high DPI, often compresses dramatically, because there was a lot of redundant pixel data to begin with.

For documents that will only ever be read on screen — emailed reports, web downloads, internal drafts — aggressive compression is almost always safe, since screen resolution can't display the extra detail anyway. For documents headed to print, especially anything with photos or fine detail, lighter compression preserves the image quality that becomes visible on paper. For scanned legal or archival documents, it's worth checking a compressed sample at full zoom before replacing the original, since over-compression can blur small text inside a scanned image in a way that's hard to reverse.