.PDF File Format – Versions

PDF 1.5 (2003)

• Object streams and cross-reference streams, and hybrid files with XRefStm: the change that makes this a separate format from PDF 1.0 to 1.4.
• JPEG 2000 images, with SMaskInData, and 16-bit image samples.
• Crypt filters: encryption V 4 and R 4, CF, StmF, StrF, the Crypt filter and EncryptMetadata; the public-key format adbe.pkcs7.s5.
• Optional content (layers): OCProperties, groups, membership dictionaries and SetOCGState.
• XFA forms in AcroForm, rich-text fields, comb fields and RadiosInUnison.
• Multimedia renditions, the Screen annotation and the Rendition action; the Polygon, PolyLine and Caret annotations, and annotation replies.
• Permission signatures (Perms, DocMDP and usage rights), the Legal attestation, and field locks and seed values.
• FontFamily, FontStretch and FontWeight in font descriptors; UTF-16 CMaps.
• The TwoPageLeft and TwoPageRight layouts, the UseOC page mode, and the page keys Tabs, TemplateInstantiated and PresSteps.

PDF 1.6 (2004)

• AES-128 encryption: the AESV2 crypt filter method, and EFF for embedded files.
• OpenType fonts embedded in FontFile3 with Subtype OpenType.
• 3D annotations, with U3D streams, and the GoTo3DView action.
• NChannel colour spaces, a kind of DeviceN.
• UserUnit for very large pages, and viewports (VP) with measurement.
• The Watermark annotation, the GoToE action for embedded PDFs, the UseAttachments page mode and PrintScaling.
• Visibility expressions (VE) and locked layers in optional content.

PDF 1.7 (2006) and ISO 32000-1:2008

• Portable collections (Collection): PDFs that are folders of attachments.
• The Redact annotation.
• UF, Unicode file names, in file specifications.
• The Extensions dictionary and Requirements.
• Print settings in viewer preferences: Duplex, PickTrayByPDFSize, PrintPageRange, NumCopies.
• NeedsRendering, for dynamic XFA forms.

Adobe gave the PDF 1.7 Reference to ISO, which published it as ISO 32000-1 on 1 July 2008. Its content is PDF 1.7, with the text rewritten in the language of a standard. A conforming PDF 1.7 file is a conforming ISO 32000-1 file. Adobe’s copy of the standard stays free to read.

Adobe’s extension levels

Between ISO 32000-1 and PDF 2.0, Adobe extended PDF 1.7 in its own supplements. A file that uses one says so in the catalog: /Extensions <</ADBE <</BaseVersion /1.7 /ExtensionLevel 3>>>>.

Most of level 3 and level 8 went into PDF 2.0. XFA did not.

PDF 2.0 (2017 and 2020)

ISO 32000-2:2017 was published in July 2017, the first PDF written by ISO without Adobe’s Reference behind it. The second edition, ISO 32000-2:2020, came out in December 2020 with corrections, and is the one in force. Since 5 April 2023 the PDF Association has offered it free of charge, with its sponsors Adobe, Apryse and Foxit. Files say %PDF-2.0.

What PDF 2.0 adds:

• UTF-8 text strings, with the mark EF BB BF.
• Revision 6 of the standard security handler and AESV3 as the standard encryption.
• PAdES signatures (ETSI.CAdES.detached), document timestamps (ETSI.RFC3161) and the document security store (DSS).
• Associated files (AF) on the document, pages and objects, and the unencrypted wrapper document.
• Page-level output intents, black point compensation (UseBlackPtComp) and the halftone origin (HTO).
• Document parts (DPartRoot, DPart, GoToDp), as PDF/VT uses.
• Structure namespaces, a revised set of standard structure types, structure destinations, and pronunciation hints.
• Rich media, projection annotations, 3D measurement and geospatial features, from Adobe’s extension level 3.
• An optional length for inline images, and a required appearance stream for most annotations.

What PDF 2.0 deprecates, which new files should no longer use and readers must still read:

• Every information dictionary key except CreationDate and ModDate, in favour of XMP.
• RC4, AES-128 and revisions 2 to 5 of the standard handler; Length in encryption dictionaries.
• XFA forms, NeedAppearances and NeedsRendering.
• Sound and Movie annotations and actions, TrapNet annotations.
• ProcSet, the Name of fonts and XObjects, OPI, Web Capture keys, transfer functions, CharSet and CIDSet, arrays of blend modes, and external data in cross-reference streams.

PDF 2.0 also requires the trailer’s ID in every file, and widths and font descriptors for the standard 14 fonts.

ISO extensions to PDF 2.0

A file that uses one says so in its Extensions dictionary under the prefix ISO_.

PDF/A, PDF/X, PDF/UA, PDF/E and PDF/VT

Each of these ISO standards takes a version of PDF and sets rules for one use: what a file must contain and what it may not. A file that meets one is still an ordinary PDF, with a mark in its XMP metadata.