PDF / A is a format for long-term archiving of digital documents, which has been standardized by the International Organization for Standardization (ISO) as a subset of the Portable Document Format ( PDF). The standard specifies how the elements of the underlying PDF versions must be used with regard to the long-term archiving. There are both mandatory and non- approved ingredients.
The standard was established in 2005 as ISO 19005-1:2005, Document management - Part 1 - Electronic document file format for long -term preservation: published Use of PDF 1.4 ( PDF/A-1 ) and specifies two levels of conformance:
- PDF/A-1b - Level B ( Basic) compliance: exact visual reproduction
- PDF/A-1a - Level A ( Accessible ) compliance: in exact visual reproduction as well as mapping text to Unicode and structuring of the document so that it can be read in terms of the accessibility of a screen reader.
- PDF/A-1 based on PDF 1.4.
- Properties of PDF/A-1b: Not allowed are references to resources that are not included in the file itself, which means in particular that all images and fonts used ( the limit of the characters used is allowed) must be included in the file.
- Colors must be sufficiently defined to ensure a unique color representation. Here either source profiles or "Output Intent" come ( description of the typical type of output using an ICC profile, such as sRGB for screen-based documents) to the application.
- Transparent elements are not allowed.
- Encryptions and thus partially blocking functions such as printing and file data copy is prohibited.
- The use of proprietary components, in particular the compression by means of Lempel -Ziv -Welch algorithm ( LZW ) is prohibited. Although it can be assumed that the LZW compression algorithm is now more burdened with any patent, was by the ISO but for safety reasons the use of LZW excluded - not least because ZIP is an equal compression available that is free of patents.
- Properties of PDF/A-1a, in addition to the properties of PDF/A-1b: All text must be able to be mapped to Unicode.
- The content structure of the PDF / A file must be specified using tagged PDF.
The standard was on 20 June 2011 and ISO 19005-2:2011, Document management - Part 2 - Electronic document file format for long -term preservation: Use of ISO 32000-1 ( PDF/A-2 ) and has defined three levels of conformance:
- PDF/A-2a: fully realized all the requirements of ISO 19005-2, and in particular all the structural and semantic properties.
- PDF/A-2b: Minimum requirement of a PDF/A-2 file, guarantees the correct appearance of the document for long-term archiving.
- PDF/A-2u: as 2b, plus: all text is displayed in Unicode so that all the text can be indexed and displayed.
- Key enhancements over PDF/A-1: based on PDF 1.7 ( ISO 32000-1 )
- JPEG 2000 compression allows
- Transparent elements are allowed
- Permitted levels
- OpenType fonts can be embedded
- Digital signatures in accordance with the PAdES (PDF Advanced Electronic Signatures, ETSI TS 102 778 )
- Container: PDF/A-1 files can be embedded in PDF/A-2 files
The specification of PDF/A-3 was published on 17 October 2012.
PDF/A-1 shall continue in effect. PDF/A-1-konforme files also meet the requirements of the corresponding PDF/A-2-Konformitätslevels. Where PDF/A-1-Funktionen enough, there is no compelling reason to switch to PDF/A-2.
A validation of valid PDF / A is appropriate inspection tools (see links).