Why Does PDF Compression Make Text Blurry?

Does the compressor rasterize whole pages?

Yes. This is the key thing to understand. Rather than selectively re-compressing the images already inside the PDF, the tool renders every page to a canvas at a chosen resolution and re-encodes that canvas as a JPEG, then assembles those JPEGs into the output PDF. Sharp vector text — which is normally resolution-independent — becomes a grid of pixels at whatever resolution you picked. Drop the resolution and the letter edges soften; raise the JPEG compression and you also get faint blocky artefacts around high-contrast edges, which is exactly where black text meets a white page.

Which resolution and quality settings are used?

The presets trade page resolution (measured in DPI) and JPEG quality against file size. Lower numbers mean a smaller file and softer text.

• ·Medium — around 200 DPI at high JPEG quality. Text stays clearly legible on screen; this is the usual starting point.
• ·High — around 150 DPI at moderate quality. Noticeably softer text, acceptable for reference copies and email.
• ·Maximum — around 96 DPI at low quality. The smallest file the tool produces and the most visible loss; fine line detail and small type degrade.

Why did my searchable text stop being selectable?

Because the compressed pages are now images, the text characters no longer exist as text — they are pixels in a JPEG. You cannot select, copy, or search the compressed output, and a screen reader cannot read it. If the recipient needs to work with the text, send the original and mention the file size instead, or compress a copy and keep the searchable original for yourself.

Which documents are poor candidates for this kind of compression?

• ·Contracts, reports, and other text-dense pages — they have little to gain from image compression and the most to lose in legibility.
• ·Documents with fine line art, tables with thin rules, or small footnote type — thin features blur first.
• ·Files people will search, copy from, or read with assistive technology — rasterizing removes the text layer entirely.
• ·PDFs that are already small. If the file is a few hundred kilobytes of mostly text, re-rendering it as images can make it larger, not smaller.

Scanned documents are the opposite case: they are already page images, so compression mainly re-encodes what is there and the relative quality loss is smaller. If a scan is unreadable after compression, the source scan resolution was probably low to begin with.

What should you inspect after compressing?

• ·Read the smallest body text at 100% zoom — if you would not want to read the whole document at that quality, step the preset back up.
• ·Check any thin lines, table borders, and signatures for breaks or fuzziness.
• ·Try to select a word. If you need selectable text and cannot get it, this compression method is not the right fit.
• ·Confirm the file actually got smaller, and by enough to be worth the quality trade.

When should you keep the original instead?

Keep the original whenever the document will be edited, searched, printed at full size, or read by anyone who needs sharp small text — and compress a duplicate only for the specific place that imposes a size limit. If even the Maximum preset does not get you under the limit, the more effective move is usually to reduce what is in the file: split the document with a page tool and submit only the required pages, or convert genuinely image-only content separately, rather than compressing an over-long document as hard as possible.

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