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What is the best PDF viewer that can make scanned PDFs searchable?

**Nature of PDFs**: PDFs can be categorized mainly into two types—image-based and text-based.

Image-based PDFs (like scanned documents) cannot have text selected or searched since they contain photographs of text rather than actual text.

**Optical Character Recognition (OCR)**: OCR is a technology that converts different types of documents, such as scanned paper documents, PDFs, or images taken by a digital camera into editable and searchable data.

It uses machine learning and pattern recognition algorithms to identify characters.

**How OCR Works**: The process requires identifying light and dark shades to distinguish individual characters.

The software analyzes the shapes of characters and compares them to known characters stored in its databases.

**Multi-language Support**: Advanced OCR tools can recognize multiple languages and different font types within the same document, making them incredibly versatile for users needing to work with international documents.

**Text Recognition Accuracy**: The accuracy of OCR varies depending on several factors; higher quality scans or images lead to better results.

Factors such as font type, image quality, and even the layout of the document itself can impact recognition success.

**PDF/A Standard**: Making scanned PDFs searchable involves adhering to the PDF/A specification, a standard for long-term archiving of electronic documents.

PDF/A ensures that the document will look the same and remain searchable in future software environments.

**Artifact Removal**: Many OCR tools can enhance the quality of a scanned PDF by removing artifacts, noise, and skewed text, which improves the overall output and enhances searchability.

**Hybrid PDFs**: The term "hybrid PDF" refers to files that embed both images and searchable text.

This format allows users to perform text searches while preserving the visual fidelity of the original scanned document.

**Intelligent Character Recognition (ICR)**: Some advanced OCR systems integrate ICR, enabling the software to recognize handwritten text as well.

While still less accurate than printed text recognition, this opens possibilities for processing forms and notes.

**PDF Compression**: After applying OCR, some tools offer options to compress the resulting searchable PDF.

This compression is achieved by optimizing the embedded images and removing excess metadata, making the file size smaller without sacrificing quality.

**Cloud-based OCR**: Many modern OCR solutions are cloud-based, providing users with the ability to upload files for processing on remote servers, facilitating easy access from any device without requiring high local processing power.

**OCR in Computing History**: The development of OCR technology dates back to the early 20th century, with significant advancements during and after World War II, driven by the need to automate document processing.

**Machine Learning in OCR**: Recent advancements in machine learning have significantly enhanced OCR capabilities, allowing software to learn from new fonts and styles, becoming more accurate over time through iterative training.

**Accessibility Features**: Searchable PDFs created through OCR also improve accessibility for visually impaired individuals, who may rely on screen readers that benefit from text being recognized as actual text rather than an image.

**Open Source OCR Libraries**: There are several open-source libraries available for OCR, like Tesseract, which allow developers to integrate OCR capabilities into custom applications, broadening access to this important technology.

**Real-time OCR**: Some cutting-edge OCR applications offer real-time text recognition through mobile devices, enabling users to point their camera at a document and immediately convert the text into a digital format.

**Data Extraction Capabilities**: Advanced OCR systems can not only make PDFs searchable but can also extract structured data from forms, invoices, and tickets through methods like template-based recognition or machine learning.

**Integration with Document Management Systems**: These searchable PDFs easily integrate into document management systems, allowing users to organize, retrieve, and manage documents efficiently.

**Security Features**: Some OCR tools provide options for encryption and secure data handling, ensuring that scanned documents are not only searchable but also protected from unauthorized access.

**Licensing Models**: While some PDF viewers and OCR software are commercial, many free and open-source alternatives exist, enabling users to choose tools based on their needs without incurring costs.

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