You receive a PDF with a scanned contract, certificate, or form. You try to select some text and nothing happens — the cursor behaves as if the page is a picture. That is exactly what it is: a scanned image with no text layer underneath. OCR (optical character recognition) adds that layer and turns the image into a working document where text can be selected, copied, and searched.
When you need OCR
OCR is useful when you have a scanned PDF and want to:
- copy a passage without retyping it by hand;
- search for a word in the document with Ctrl+F;
- extract content into another tool without manual entry;
- make an archived document searchable by keyword.
If the PDF already has a text layer — meaning you can select words — OCR is not needed.
What affects recognition quality
The quality of the output is almost entirely determined by the source scan. The key factors:
- **Resolution**: good OCR needs at least 150–200 dpi, ideally 300 dpi. Phone photos are often sufficient if the page is flat and the lighting is even.
- **Alignment**: the page should be straight. Heavily tilted text recognizes poorly.
- **Lighting and contrast**: shadows from the spine, yellowed paper, or faded ink all reduce accuracy.
- **Language**: the algorithm needs to know which language the text is in to correctly interpret letters and words.
How to run OCR, step by step
1. Open ocr-pdf and upload your scanned PDF. 2. Select the document language if the tool prompts for it. 3. Wait for processing — multi-page scans take longer. 4. Download the result. 5. Open the file and verify: can you select text? Are key details — names, dates, numbers — recognized correctly?
What can go wrong
- **Many errors in names and numbers.** The scan quality is too low. Try rephotographing or rescanning at a higher resolution.
- **Text is not recognized at all.** The PDF may contain vector graphics that look like a scan but technically are not. OCR cannot help with that.
- **The file got much bigger.** OCR adds a text layer and slightly increases size. If the result is too heavy to send, compress it with **compress-pdf**.
- **Handwriting is not readable.** Standard OCR targets printed text. Handwritten signatures and margin notes recognize poorly or not at all.
What to check after OCR
- Select a few lines and paste them into a text editor — confirm the text came through correctly.
- Search for key words with Ctrl+F: a name, an organization, a date.
- Compare numbers and dates in the recognized text against the original — those are the most common source of errors.
To make a scanned PDF searchable and selectable, upload it to **ocr-pdf**. For long documents, split the file with **split-pdf** first, process the parts separately, and combine the results with **merge-pdf**.