File extension
.pdf
Portable Document Format (Adobe / ISO)
.pdf is primarily used for Portable Document Format documents: fixed-layout pages for publishing, exchange, print workflows, forms, and preservation.
Known meanings
Formats and reference material for .pdf
Portable Document Format (Adobe / ISO)
Fixed-layout document format standardized through ISO 32000; current base standard: ISO 32000-2:2020, PDF 2.0
Adobe PDF Reference, version 1.6
Adobe PDF Reference, version 1.6
Adobe PDF Reference, version 1.7
Adobe PDF Reference, version 1.7
Adobe PDF Reference, version 1.3
Portable Document Format Reference Manual Version 1.3, March 11, 1999
Details
Format notes
Portable Document Format (Adobe / ISO)
PDF is a fixed-layout document format designed to preserve page appearance across software, printers, and operating systems. PDF originated with Adobe, but the base format is now standardized through ISO 32000. The current base standard is ISO 32000-2:2020, PDF 2.0. The format matters because it is a publication format, an exchange format, a print workflow format, a forms container, and a long-term preservation target.
Object Model
A PDF file is organized as numbered indirect objects. Objects may describe page trees, fonts, images, vector graphics, annotations, metadata, forms, embedded files, encryption dictionaries, and content streams. A page usually references resources such as fonts and images, then points to one or more content streams that contain drawing operators.
PDF readers locate objects through a cross-reference table or cross-reference stream. The trailer dictionary points to the document catalog, which acts as the root of the object graph. Later PDF versions allow object streams and compressed cross-reference streams, which reduce file size but make manual inspection more difficult.
Incremental Updates
PDF supports incremental saving. Instead of rewriting the whole file, an editor can append new objects, a new cross-reference section, and a new trailer. This is useful for signatures, annotations, and workflow edits, but it means older content may remain physically present in the file even when it is no longer visible. Redaction tools must remove sensitive content, not merely cover it with new drawing instructions.
Identification
Most PDF files begin with a header such as %PDF-1.7 or %PDF-2.0.
Binary-safe PDFs commonly include a comment line containing non-ASCII bytes near
the beginning of the file so transfer systems do not mistake the document for
plain text. The end of the file normally contains %%EOF, but robust readers
often tolerate trailing bytes or damaged offsets because PDFs are common in
email, web, and scan workflows.
Preservation Profiles
Not every valid PDF is suitable for long-term preservation. PDF/A profiles vary by part and conformance level, but broadly restrict features that create external dependencies, active behavior, or uncertain rendering, such as unembedded fonts, encryption, executable actions, and device-dependent color. For archival use, prefer a validated PDF/A profile, embedded fonts, explicit metadata, stable color management, and accessible structure when possible.
Security Notes
PDF is complex enough to be a frequent attack surface. Risky features include JavaScript, launch actions, embedded files, rich media, malformed streams, and parser differences between readers. Safe processing pipelines should use well-maintained libraries, disable active content where possible, and treat rendering, text extraction, and validation as separate tasks.
Further Reading
- ISO 32000-2:2020:
https://www.iso.org/standard/75839.html - PDF Association specification archive:
https://pdfa.org/resource/pdf-specification-archive/ - Related profiles: PDF/A, PDF/X, PDF/UA, PDF/E