Organising files

Shrinking a PDF the built-in way

Updated 2026-08-08 · 4 min read

A ten-page scan comes out at 45 MB, the upload portal caps files at 10, and the obvious fix — Preview's Reduce File Size — turns crisp pages into fog. The filter is right; its default settings are wrong.

The built-in route

Open the PDF in Preview, File → Export…, and choose Reduce File Size under Quartz Filter. For PDFs bloated by high-resolution photos, this alone can cut 90% of the size. The cost: the stock filter resamples images hard, and dense scans come out visibly soft.

Why some PDFs will not shrink

A PDF's size is almost always its images. Text, fonts and vector graphics are already compact. So:

• Scanned pages (each page one big image) — shrink dramatically.
• Digital documents with photos — shrink well.
• Pure text PDFs — barely shrink; there is nothing to compress.

If Export barely moves the number, the file is already efficient.

A better filter than the default

The stock filter's image quality is a known complaint. ColorSync Utility can build a gentler one: open it, duplicate the Reduce File Size filter, and raise the image resolution and quality in the duplicate (150–200 DPI reads as clean for documents; the default resamples far lower). The custom filter then appears in Preview's Export dialog alongside the stock one.

For scans specifically

Rescanning at 200 DPI in black-and-white text mode produces a smaller, sharper file than compressing a 600 DPI colour scan after the fact. The best compression is not scanning detail you do not need.

Batch work

The same Quartz filters apply through the command line via sips, or in an Automator/Shortcuts workflow with the Apply Quartz Filter to PDF action — select thirty PDFs, right-click, done. Test on copies: filters overwrite in some configurations, and there is no undo on a re-compressed original.

Common questions

Why is my exported PDF blurry?

The stock Reduce File Size filter resamples images to a low resolution. Build a gentler copy of it in ColorSync Utility and export with that.

Why did Reduce File Size barely shrink my PDF?

The file is mostly text and vectors, which are already compact. Only image-heavy PDFs shrink substantially.