Scanned PDF versus digital PDF: why compression behaves differently
Two PDFs that look identical on screen can have completely different internal content. That difference determines which compression method is useful.
A scanned PDF is usually a stack of images
A scanner or phone camera records every page as pixels. Text may look sharp, but unless optical character recognition was added, the PDF may not contain real selectable text.
Reducing image dimensions or quality can save substantial space. It can also blur small characters and make OCR, stamps, or codes less reliable.
A digital PDF contains structured objects
A document exported from a word processor may contain text instructions, vector lines, fonts, links, and a few images. Those elements are often compact and remain sharp at any zoom level.
Lossless optimization is usually the safer first choice. Rendering the page to reduce size can sacrifice selectable text and interactive features.
How to identify the type
Mixed PDFs are common. Review every page because a report may contain digital text alongside scanned appendices.
- Try selecting one sentence. A full-page selection rectangle suggests an image.
- Zoom in closely. Pixelated letters indicate a scan; crisp letters usually indicate text or vectors.
- Search for a known word. Failure may mean the scan has no OCR layer.
- Check whether every page has a similar large size.
How to reduce PDF size without losing important readability
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