PDF/A format description
PDF/A is an ISO-standardized version of the Portable Document Format (PDF) specialized for the digital preservation of electronic documents.
PDF/A differs from PDF by prohibiting features ill-suited to long-term archiving, such as font linking (as opposed to font embedding).
Description
The PDF/A standard identifies a "profile" for electronic documents that ensures the documents can be reproduced exactly the same way using various software in years to come.
A key element to this reproducibility is the requirement for PDF/A documents to be 100% self-contained. All of the information necessary for displaying the document in the same manner is embedded in the file.
This includes, but is not limited to, all content (text, raster images and vector graphics), fonts, and color information.
A PDF/A document is not permitted to be reliant on information from external sources (e.g. font programs and data streams), but may include annotations (e.g. hypertext links) that link to external documents. Other key elements to PDF/A conformance include:
Audio and video content is forbidden.
JavaScript and executable file launches are forbidden. All fonts must be embedded and also must be legally embeddable for unlimited, universal rendering. This also applies to the so-called PostScript standard fonts such as Times or Helvetica.
Colorspaces specified in a device-independent manner.
Encryption is forbidden.
Use of standards-based metadata is required.
External content references are forbidden.
Transparent objects and layers (Optional Content Groups) are forbidden in PDF/A-1, but are allowed in PDF/A-2.
Provisions for digital signatures in accordance with the PAdES (PDF Advanced Electronic Signatures) standard are supported in PDF/A-2.
Embedded files are forbidden in PDF/A-1, but PDF/A-2 allows embedding of PDF/A files, facilitating the archiving of sets of PDF/A documents in a single file. PDF/A-3 allows embedding of any file format such as XML, CAD and others into PDF/A documents.
The use of XML-based XML Forms Architecture (XFA) forms is forbidden in PDF/A. (XFA form data may be preserved in a PDF/A-2 file by moving from XFA key to the Names tree that itself is the value of the XFAResources key of the Names dictionary of the document catalog dictionary.)
Interactive PDF forms - Form fields must have an appearance dictionary associated with the field's data. The appearance dictionary shall be used when rendering the field.
PDF/A support in Seascape
SWING Seascape supports multiple PDF/A output formats, those include:
PDF/A-1b
PDF/A-2b
PDF/A-3b
Notes document links remain functional in all three supported PDF/A formats, however the attachment embedding feature works only with PDF/A-3b format.
PDF/A-1b
A-1b conformance is the minimum requirement for PDF/A compliance. The focus here is on reliable rendered visual appearance. PDF/A-1b files must include:
Embedded fonts
Device-independent color
Extensible Metadata Platform (XMP) metadata
PDF/A-1b files can not include:
Encryption
LZW Compression
Embedded files
External content references
PDF Transparency
Multi-media
JavaScript
PDF/A-2b
PDF/A-2b is the extension of the PDF/A1b conformance. Part 2 of the PDF/A Standard is based on a PDF 1.7 (ISO 32000-1), rather than PDF 1.4 and offers a number of new features:
JPEG2000 image compression.
Support for transparency effects and layers.
Embedding of OpenType font.
Provisions for digital signatures in accordance with the PDF Advanced Electronic Signatures – PAdES standard.
Embedding PDF/A files to facilitate archiving of sets of documents with a single file.
PDF/A-3b
PDF/A-3 (ISO 19005-3:2012. Part 3) differs from PDF/A-2 in only one regard - it allows embedding of arbitrary file formats (such as XML, CSV, CAD, word-processing documents, spreadsheet documents and others) into PDF/A conforming documents.
References
1. Wikipedia - https://en.wikipedia.org/wiki/PDF/A
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