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About PDF Format
The Portable Document Format (PDF), standardized as ISO 32000, is a file format developed by Adobe Inc. to present documents, including text formatting and images, in a manner independent of application software, hardware, and operating systems. Based on the PostScript language, PDF files encapsulate a complete description of a fixed-layout flat document, ensuring consistent display across platforms.
- Filename extension: .pdf
- Internet media type: application/pdf, application/x-pdf, application/x-bzpdf, application/x-gzpdf
- Type code: PDF (including a single trailing space)
- Uniform Type Identifier (UTI): com.adobe.pdf
- Magic number: %PDF
- Developed by: Adobe Inc. (1991–2008), ISO (2008–present)
- Initial release: June 15, 1993
- Latest release: 2.0
- Standard: ISO 32000-2
- Open format: Yes
- Website: https://iso.org/standard/75839.html
PDF files can include text, vector graphics, raster images, and other content such as logical structuring elements, interactive annotations, form-fields, layers, rich media (including video), three-dimensional objects, encryption, digital signatures, file attachments, and metadata, supporting various workflows.
History and Development
PDF's development began in 1991 with "The Camelot Project," initiated by Adobe co-founder John Warnock, proposing a simplified version of PostScript called Interchange PostScript (IPS) optimized for screen display across platforms. Adobe made the PDF specification available free of charge in 1993, initially popular in desktop publishing workflows, competing with formats like DjVu and Envoy.
- Initially a proprietary format controlled by Adobe, PDF became an open standard on July 1, 2008, published as ISO 32000-1:2008, with control passing to an ISO Committee.
- Adobe published a Public Patent License in 2008, granting royalty-free rights for PDF-compliant implementations.
- PDF 1.7, which became ISO 32000-1, included some proprietary technologies like Adobe XML Forms Architecture (XFA) and JavaScript extensions, not fully supported by third-party implementations.
- ISO published PDF 2.0 (ISO 32000-2) in 2017, with the second edition (ISO 32000-2:2020) released in December 2020, excluding proprietary technologies as normative references. It became freely available for download in April 2023.
Features and Technical Aspects
PDF combines vector graphics, text, and bitmap graphics, supporting content streams for typeset text, vector illustrations, raster images, and multimedia objects. It includes font-embedding for portability, a structured storage system with data compression, and supports links, forms, JavaScript, and embedded content via plug-ins.
- PostScript Basis: PDF is a subset of PostScript, simplified to remove control flow features, focusing on declarative layout and graphics, unlike PostScript's programming capabilities.
- Advantages over PostScript: Includes static declarative code, supports transparent graphics since version 1.4, ensures page independence, and bundles all rendering data for portability, though it may result in larger file sizes.
- 3D Support: Since version 1.6, PDF supports embedding interactive 3D documents using U3D or PRC formats.
- File Structure: Organized using ASCII characters, with a header starting with %PDF, containing objects like Booleans, numbers, strings, arrays, dictionaries, streams, and the null object, supporting efficient random access via a cross-reference table.
- Linearized PDFs: "Optimized" or "web optimized" PDFs are structured for faster web browser loading, with first-page data at the file start.
- Page Dimensions: Adobe Acrobat limits pages to 15 million by 15 million inches, covering an area slightly larger than Tajikistan.
The imaging model uses a device-independent Cartesian coordinate system, supporting scaling, rotation, and skewing of graphical elements, with graphics state properties for rendering. It includes vector graphics (paths, lines, Bézier curves), raster images (with compression filters like FlateDecode, JPEG, JBIG2), and text (using font objects with embedded or unembedded fonts, supporting standard encodings like WinAnsi and MacRoman).
- Transparency: Added in PDF 1.4, supporting blending effects through transparency groups and modes, compatible with older viewers for limited use.
- Logical Structure: Tagged PDFs include structure and semantics for accessibility and reflow.