如何在线使 PDF 可搜索 — 免费 OCR 指南

An image-only PDF stores each page as a raster image (pixels). A searchable PDF has two layers: the visible image of the page, plus an invisible text layer underneath it. When you press Ctrl+F, your PDF viewer searches this text layer — which is why a scanned PDF without OCR returns zero results.

1. Go to WhiskFile's OCR PDF tool 2. Upload your scanned PDF 3. Select your document language (affects OCR accuracy) 4. Click 'Run OCR' 5. Download the resulting searchable PDF

The output is visually identical to the original but now has a searchable text layer. You can immediately Ctrl+F through the document.

OCR accuracy depends heavily on scan quality:

- **Scan at 300 DPI minimum** — lower resolution produces blurry text that OCR misreads - **Deskew before OCR** — tilted pages reduce accuracy. Use WhiskFile's Deskew PDF tool first - **High contrast** — black text on white background gives the best results - **Avoid handwriting** — OCR is optimised for printed text; handwriting accuracy is much lower - **Select the correct language** — mixing languages in one document reduces accuracy on the minority language

You can verify by pressing Ctrl+F in your PDF viewer and searching for a word you can see. If it highlights, your PDF is already searchable and does not need OCR.

If Ctrl+F finds nothing (or gives zero results for common words), the PDF is image-only and needs OCR.