.PDF File Format

Portable Document Format, PDF 1.5 to 2.0

Introduction

This is PDF from version 1.5, of Acrobat 6.0 in 2003, to PDF 2.0, the version in use today: the files written by Acrobat 6.0 and later and by nearly every program that writes PDF now. A PDF holds fixed pages of text, vector graphics and images, with the fonts it needs, and can carry links, forms, annotations, digital signatures, attachments, layers, 3D models and accessibility tags. PDF 1.7 became the international standard ISO 32000-1 in 2008, and PDF 2.0 is ISO 32000-2, first published in 2017, with a second edition in 2020. Documents are made and read with Adobe Acrobat and many other programs.

PDF 1.5 changed how objects are stored and found. Objects may be packed, many at a time, inside a compressed object stream, and the cross-reference table may be replaced by a cross-reference stream: a compressed binary index that can point into those object streams. A reader of PDF 1.4 cannot find such objects, so this is a separate format from PDF 1.0 to 1.4. Hybrid files keep a plain table as well, for older readers. Versions 1.6, 1.7 and 2.0 add to this format and deprecate parts of it, but do not change these structures, so they belong to it. PDF 1.6 brought AES encryption, and Adobe’s extensions to 1.7, then PDF 2.0, AES with 256-bit keys.

A .pdf file dropped on any page opens in the explorer, which shows its objects, unpacks object streams, decodes cross-reference streams, walks the page tree and decodes the other streams, with fonts, images and other embedded files opening in their own viewers. It does not draw the pages.

Structure

A file is a header line, a body of objects, a cross-reference section and a trailer. The cross-reference section is either a plain xref table with a trailer dictionary, or a cross-reference stream whose dictionary is the trailer. startxref at the end gives its offset either way.

  1. 0Header %PDF-1.5 to %PDF-1.7, or %PDF-2.0, and a binary commenttwo lines
  2. …Body indirect objects and object streams that hold more objectsmost of the file
  3. …Cross-reference stream /Type /XRef: binary entries for every object, and the trailer’s keys; or an xref table and trailera few bytes an object
  4. …startxref the offset of the cross-reference section, and %%EOFthree lines
The parts of a PDF 1.5 to 2.0 file as first written; an update appends a body, a cross-reference section and %%EOF

A real file

A small PDF 1.7 file of 3,527 bytes. Its header and binary comment come first, then object 3:

00000000  25 50 44 46 2D 31 2E 37 0A 25 E4 E3 CF D2 0A 33  %PDF-1.7.%.....3
00000010  20 30 20 6F 62 6A 0A 3C 3C 2F 4C 65 6E 67 74 68   0 obj.<</Length
%PDF-1.7, a comment of four bytes above 127, and 3 0 obj at offset 15

Five of its objects are packed in object stream 10, and the file ends with a cross-reference stream, object 16, instead of a table:

10 0 obj
<</Length 15 0 R/Filter/FlateDecode/Type/ObjStm/N 5/First 27>>
…
16 0 obj
<</Filter/FlateDecode/Type/XRef/W[1 2 2]/Size 17/Root 14 0 R/Info 11 0 R
/ID[<57A984D8D8017E7021C18DB41CC7EE07><57A984D8D8017E7021C18DB41CC7EE07>]
/DecodeParms<</Columns 5/Predictor 12>>/Length 87        >>
stream
…87 bytes…
endstream
endobj
startxref
3180
%%EOF
An object stream of five objects, and a cross-reference stream with 5-byte entries (W [1 2 2]) under the PNG Up predictor

From the trailer to a page

The trailer’s Root, here in the cross-reference stream’s dictionary, is the catalog, whose Pages is the root of the page tree. Each page has a MediaBox, its Resources and its Contents, a content stream of drawing operators. The page dictionaries themselves may be inside an object stream, so a reader finds them through the cross-reference stream’s type 2 entries.

Reference pages

  • File structure – the header, binary comment, body, cross-reference section, trailer, %%EOF, incremental updates and hybrid files
  • Objects and syntax – the eight object types, strings, names, indirect objects and references, text strings and dates
  • Streams and filters – the stream dictionary, every filter of these versions, Crypt filters and the predictors
  • Cross-reference and trailer – xref tables, cross-reference streams and their entry types, object streams, hybrid-reference files and the trailer
  • Document structure – the catalog, page tree, pages, resources, name and number trees, outlines, destinations, structure, optional content and collections
  • Content streams – all 73 operators with their operands and the version that added each
  • Graphics – coordinate spaces, colour spaces, patterns, shadings, functions, graphics state and transparency
  • Fonts – Type 1, TrueType, Type 3, Type 0 and CIDFonts, encodings, CMaps, ToUnicode, descriptors and embedding, OpenType included
  • Images – image XObjects, Decode arrays, masks and soft masks, JPEG 2000 and inline images
  • Interactive features – annotations, actions, JavaScript, forms, signatures, embedded files and multimedia
  • Metadata – the document information dictionary, XMP metadata streams and what PDF 2.0 changes
  • Encryption – crypt filters, AES-128 and AES-256, the standard security handler revisions 2 to 6, permissions and public-key handlers
  • Linearization – fast web view: the parameter dictionary, the order of parts, the hint tables, and object streams
  • Versions – 1.5, 1.6, 1.7, Adobe’s extension levels, 2.0, and PDF/A, PDF/X, PDF/UA and PDF/E

Telling it apart

A PDF starts with %PDF- (25 50 44 46 2D) and a version. A file is in this format when its header says 1.5, 1.6, 1.7 or 2.0, or when, whatever its header says, it holds a cross-reference stream (/Type /XRef), an XRefStm entry in a trailer, or an object stream (/Type /ObjStm). Some programs write a 1.4 header and then add a cross-reference stream when they update the file; such a file is read as this format.

A file with a header of 1.0 to 1.4 that has only plain tables and no object streams is the older format, PDF 1.0 to 1.4. The catalog’s Version entry can raise the version above the header’s, and PDF 2.0 files written by updating older ones often do so.

Some files with other extensions are PDF inside. Adobe Illustrator’s .ai files from Illustrator 9 on are PDF files that also carry Illustrator’s own data, and open in a PDF reader as their artwork.

Variants

Links

Software

  • Adobe Acrobat Reader – Adobe’s free PDF viewer, with forms and comments (Windows, macOS, mobile)
  • PDF.js – the viewer built into Firefox (web browsers)
  • SumatraPDF – small, fast free viewer (Windows)
  • Okular – KDE’s document viewer (Linux, Windows)
  • LibreOffice Draw – free drawing program that can edit PDF pages (Windows, macOS, Linux)
  • Ghostscript – interpreter that renders, converts and compresses PDF (Windows, macOS, Linux)
  • qpdf – command-line tool that shows a file’s structure, unpacks object streams, decrypts and repairs PDF (Windows, macOS, Linux)

Developers

  • PDF.js – library that parses and renders PDF (JavaScript)
  • pypdf – library that reads, splits, merges and writes PDF (Python)
  • Apache PDFBox – library for reading and creating PDF (Java)
  • qpdf – library for inspecting and rewriting PDF structure (C++)
  • MuPDF – library and viewer for rendering PDF (C)

Reference