What Is OCR?
OCR (Optical Character Recognition) is a technology that converts images of text into machine-readable text. When you scan a paper document, the result is an image — a photograph of the page. Even though you can see words and sentences in the image, a computer sees only pixels. OCR software analyzes the shapes of those pixels to identify individual letters, numbers, and symbols, then assembles them into words and sentences that you can edit, search, copy, and paste.
When Do You Need OCR?
You need OCR when your PDF was created by scanning paper documents using a flatbed scanner, phone camera, or multifunction printer. These scanned PDFs contain images of pages rather than actual text data. If you try to select or copy text from a scanned PDF and nothing highlights, that means the document needs OCR. Common scenarios include scanned contracts, old archived documents, photographed receipts, book pages, handwritten notes, and any document that originated as physical paper.
How ConvertKr's OCR Works
ConvertKr uses Tesseract.js, the JavaScript port of Google's open-source Tesseract OCR engine — one of the most accurate and widely used OCR engines in the world. When you upload a scanned PDF, the tool first renders each page as a high-resolution image using PDF.js. These images are then fed to the Tesseract OCR engine, which analyzes the visual patterns to recognize text. The engine uses trained language models to understand character shapes, word boundaries, and common language patterns. All of this runs entirely in your browser with no server involvement.
Tips for Best OCR Results
The quality of OCR output depends heavily on the quality of the input image. For best results, use scans at 300 DPI or higher with good contrast between text and background. Avoid skewed or rotated pages — straighten them before scanning if possible. Select the correct language in the settings, as the OCR engine uses language-specific models to improve accuracy. Clean, clearly printed text in standard fonts will produce the best results. Handwritten text, decorative fonts, and very small text may reduce accuracy.
Privacy and Security
All OCR processing happens entirely within your web browser. Your files are read into memory using the JavaScript FileReader API, rendered to images by PDF.js, and processed by Tesseract.js — all locally. The only network activity is downloading the Tesseract language data file from a CDN on first use, which is cached by your browser for subsequent sessions. At no point does your actual document data leave your device. This makes the tool safe for confidential documents, legal files, medical records, and any other sensitive content.