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How OCR Makes Scanned PDFs Searchable

By PageFlow Editorial Team · Editorial Team ·

Optical Character Recognition (OCR) is the process of turning an image of text into actual, matchable characters. For scanned PDFs — where every page is really just a picture — OCR is what makes search possible. Here’s how OCR PDF handles it, entirely on your own device.

The process, step by step

  1. Detection. When you open a PDF that has little or no extractable text, PageFlow flags it as a likely scan and offers OCR.
  2. Page and language selection. Choose which pages to process and which language to recognize (English is supported at launch; additional languages are added only once verified reliable).
  3. Rendering. Each selected page is rendered to an image at a resolution suited for recognition.
  4. Recognition. A local OCR engine analyzes each rendered page in a background worker, so your browser tab stays responsive while it works.
  5. Progress and cancellation. You see per-page progress as recognition runs, and can cancel at any point.
  6. Output. You can search the recognized text immediately within your session, or export a new PDF with a hidden, searchable text layer added behind the original page images.

Everything happens locally

The rendered page images and recognized text never leave your browser. There’s no upload step in this process — the OCR engine runs as WebAssembly/JavaScript directly on your device.

Accuracy: what to expect

OCR is genuinely useful, but it isn’t magic. Recognition quality depends heavily on:

  • Scan resolution — a crisp, well-lit scan recognizes far better than a blurry photo.
  • Handwriting — OCR is built for printed text; handwritten notes are much less reliable.
  • Rotation — sideways or upside-down pages reduce accuracy unless corrected first (see Rotate PDF).
  • Mixed languages — a single recognition pass tuned for one language will struggle with pages that mix scripts.
  • Complex tables and decorative fonts — dense layouts and stylized typefaces are the hardest cases for any OCR engine.

Treat OCR output as a strong starting point, not a guaranteed-accurate transcript — especially for anything where precision matters.

Two ways to use the result

  • Search now: once OCR finishes, the recognized text is indexed for the rest of your session, so you can search it immediately without exporting anything.
  • Export a searchable copy: PageFlow can also bake a hidden text layer into a new PDF, positioned behind the original scanned images. The page still looks exactly like the scan, but text tools, search, and copy-paste now work against it.

If you’re not sure whether your document even needs OCR, start with Why a PDF is not searchable — it explains how to tell a text PDF from a scanned one before you run anything.

Open OCR PDF