PDF/A is an ISO standard for using the PDF format for long-term archiving of electronic documents.
PDF/A-1 (ISO 19005-1) is based on PDF 1.4 (Acrobat 5). In addition to the PDF 1.4 requirements, it specifies that documents must be kept self-contained and suitable for long-term archival.
Key differences between PDF 1.4 and PDF/A-1
- Encryption may not be used.
- If device-dependent color space is used (i.e. DeviceRGB, DeviceCMYK, DeviceGray), a corresponding color profile must be embedded.
- Fonts used for visible text must be embedded.
- Transparency may not be used.
PDF/A-2 is described in ISO 19005-2. It is based on ISO 32000-1, the standard for PDF 1.7. PDF/A-2 is meant as an extension to PDF/A-1. The second part complements the first part and does not replace it.
Key differences between PDF/A-1 and PDF/A-2
- The list of compression types has been extended by JPEG2000.
- Transparent contents produced by graphic programs are allowed.
- Optional contents (also known as layers) can be made visible or invisible.
- Multiple PDF/A files can be bundled in one file (collection, package).
- The additional conformity level U (Unicode) allows for creating searchable files without having to fulfill the strict requirements of the conformity level A (accessibility).
- File size can be reduced using compressed object and XRef streams.
Any documents that contain these features (particularly, layers or transparency) should be converted to PDF/A-2 rather than PDF/A-1.
PDF/A-3 is described in ISO 19005-3. It is based on ISO 32000-1, the standard for PDF 1.7. PDF/A-3 is an extension to PDF/A-2. The third part complements the second part and does not replace it.
Key differences between PDF/A 2 and PDF/A 3
- Files of any format and conformance may be embedded. Embedded files need not be suitable for long-term archiving.
- Embed files can be associated with any part of the PDF/A-3 file.