如何在线对扫描版 PDF 进行 OCR 文字识别 — 免费指南

When you scan a physical document with a scanner or camera, the output is a photograph of the page — stored as pixels inside a PDF container. There is no actual text data, just pixels arranged to look like letters.

OCR software analyses those pixels, recognises letter shapes, and converts them into actual text characters. The result is a PDF that looks identical to the original scan but now has a hidden text layer that supports searching, copying, and indexing.

1. Go to the WhiskFile OCR PDF tool.

2. Upload your scanned PDF. Files up to 50 MB are accepted on the free plan.

3. Select the language of the document. Correct language selection improves accuracy significantly — English is selected by default but 100+ languages are supported.

4. Click "Apply OCR". Processing time depends on page count — expect 2–5 seconds per page.

5. Download the searchable PDF. Open it in any PDF viewer and try Ctrl+F — your text is now searchable.

OCR accuracy depends heavily on scan quality. For best results:

- Scan at 300 DPI or higher. Below 200 DPI, letter shapes become too blurry for reliable recognition. - Use high contrast. Black text on white background gives the best results. Faded ink, yellowed paper, or coloured backgrounds reduce accuracy. - Deskew before OCR. If pages are slightly rotated (common from flatbed scanners), use the WhiskFile Deskew PDF tool to straighten them first. Even a 2–3° tilt measurably reduces OCR accuracy. - Remove noise. Coffee stains, smudges, and scan artefacts confuse OCR engines. Professional scanning equipment handles this automatically.

WhiskFile uses Tesseract 5 OCR, which supports over 100 languages including all major European languages, Arabic, Chinese (simplified and traditional), Japanese, Korean, Hindi, and more.

Select the correct primary language before processing. For mixed-language documents, select the dominant language — Tesseract has limited multi-language support in a single pass.

The OCR PDF tool creates a searchable PDF — the output looks identical to the input but has a hidden text layer. The original scan and the text exist together.

The PDF to Text tool (with OCR enabled) extracts text only, outputting a plain .txt file with no images or layout. Use OCR PDF when you want to keep the original appearance of the document. Use PDF to Text when you only need the raw text content for processing, indexing, or import into another system.