PDF Compression Guide

Overview

PDF file size is rarely about “too many pages” alone. Most bulk comes from embedded images, inefficient image encoding, unused objects, high-resolution scans, and fonts. Effective compression means identifying which of those you can reduce without breaking the document’s purpose.

There is no single best setting. A brochure destined for offset press, a scanned contract for email, and a PDF/A records package have different tolerance for lossy image recompression. Decide the use case first, then choose how aggressive to be with downsampling and JPEG quality.

On Dockzio, the pdf-compress tool is built for practical size reduction in the browser. Pair it with judgment: preview critical pages after compression, especially those with fine type, charts, or logos that sit on photographic backgrounds.

Step-by-step

1. 1. Diagnose what is inflating the file

   Open the PDF and scan for full-page photos, scanned pages, and vector artwork that was rasterized during export. Scans at 300–600 dpi in color are frequent size offenders when the real need is readable text.

   Check whether the file was exported with unnecessary printer’s marks, multiple unused page sizes, or embedded multimedia. Removing unused content often beats crushing image quality.

2. 2. Choose a target use case and size budget

   Email attachments often need to stay under organizational limits (commonly 10–25 MB). Web previews can tolerate more aggressive downsampling. Print masters should prioritize resolution over kilobytes.

   Write down a success criterion (“under 5 MB, logos still sharp on page 1”) so you stop when quality is still acceptable instead of recompressing until artifacts appear.

3. 3. Downsample images to the resolution you actually need

   For screen reading, 100–150 ppi at final size is often enough. For office printing, 200–300 ppi is a common target. Commercial press work usually wants ~300 ppi at final size—do not casually downsample a press PDF.

   Apply stronger reduction to photographic images than to screenshots of UI or text-heavy scans; screenshots suffer visibly when JPEG artifacts hit edges.

4. 4. Run compression and verify critical pages

   Use Dockzio’s pdf-compress to produce a smaller copy, then flip through pages that contain small type, barcodes, signatures, and brand colors.

   If one page fails quality checks, consider compressing only image-heavy sections (via split/merge workflows) or re-exporting those pages from the source at saner settings.

5. 5. Preserve standards when they matter

   If the original was PDF/A or PDF/X, re-validate after compression. Size tools can strip output intents, alter color, or introduce constructs that break conformance.

   For archives, prefer lossless or lightly lossy settings and keep the uncompressed validated master separately.

Common mistakes

• Compressing a press-ready master in place. Always keep an uncompressed or lightly processed print master. Send the compressed derivative for email review if needed, not as the plate file.
• Judging quality only by file size. A 900 KB PDF can be unreadable if type was over-JPEG’d. Check zoomed text and thin lines, not only the byte count.
• Recompressing an already compressed scan. Repeated lossy compression stacks artifacts. If a scan is still too large, consider a cleaner re-scan or OCR-to-text workflow rather than another JPEG pass.
• Ignoring fonts and structure. Some “compressors” subset or rewrite fonts poorly. If text looks substituted after compression, stop and try a different setting or tool path.