Does Compressing a PDF Reduce Quality? (Honest Answer)

Does compressing a PDF reduce quality? "Without losing quality" needs a small correction: compression that makes a real difference to file size does discard something. The useful question is whether it discards anything you can see.

Does compressing a PDF reduce quality?

It depends on the kind of compression. Lossless compression — repacking the file's structure — reduces size with no change at all to text or images. Lossy compression — lowering image resolution and JPEG quality — does reduce quality, though at moderate settings the loss is invisible on screen. Big savings on large files almost always need the lossy kind.

• Lossless: no visible change; savings are usually modest.
• Lossy, moderate: invisible on screen, noticeable only when zoomed in or printed large.
• Lossy, aggressive: soft text, blocky photos, and pages that have become images.

On a scanned document, moderate lossy compression usually costs nothing you would notice — you can cut most of the size with the text still crisp. On a photographic brochure, you need to be more careful.

How Slate PDF compresses

1. 1Open the Compress PDF tool and add your file.
2. 2Type the target size in KB or MB, or pick a preset — 50%, 25%, 10% or 5% of the original.
3. 3The tool first tries a lossless repack. If that alone reaches the target, your text and images are untouched.
4. 4If not, it redraws each page as a JPEG image, starting at about 144 DPI and 92% quality and stepping down only until the file fits.
5. 5Download, then compare with the original at 100% zoom.

Because it stops at the first setting that hits your target, asking for a smaller file is the only way to lose more quality. Ask for what you actually need, not the smallest number on offer. If even the lowest setting cannot reach the target, the tool says so and shows the smallest size it managed.

What compression actually does

Nearly all of the size in a large PDF is images, so nearly all of compression is about images. In general, three things can happen:

• Downsampling: a 600 DPI image is reduced to 150 DPI, throwing away pixels you were never going to see on screen.
• Re-encoding: images are recompressed as JPEG at a lower quality setting.
• Structural clean-up: unused objects and old revisions dropped, the file repacked more tightly.

Slate PDF's lossy route is blunter than per-image downsampling: it redraws the whole page, so vector text becomes part of the image. The original text is laid back over each page as an invisible layer, so searching and copying usually still work, but the letters you see are pixels and soften as the target shrinks.

What you can expect

Check it properly

1. 1Open the compressed file next to the original at 100% zoom.
2. 2Look at the smallest text on the busiest page — that is where damage appears first.
3. 3Check any photographs for blocky artefacts in smooth areas like skies and skin.
4. 4If it will be printed, print one page. Screen quality and print quality are not the same test.
5. 5Try selecting text. If the highlight sits over an image of the words, the pages were rasterised — fine for reading, worse for editing and screen readers.

Never compress twice

JPEG damage accumulates. Compressing an already-compressed file re-encodes artefacts as if they were detail, so you lose quality for very little size. If the result is not small enough, go back to the original and compress it harder in one pass. Scans have their own tricks, covered in compressing a scanned PDF.

If you are not sure what is making the file big in the first place, start with why a PDF gets so large.