The PDF/A standard is based around specifying the features of
PDF documents which make them suitable for long term archiving.
The standards are realized in ISO 19005 "Electronic document
file format for long-term preservation". There are now four
editions - PDF/A Part 1 to PDF/A Part 4.
There are variants of each standard tagged either B (basic) or
A(accessible), so an A specification typically builds on a B
standard rather than the other way round. As well as A and B
standards there are also U standards which specify Unicode text
PDF/A Part 1 Level B conformance mandates features like font
embedding to ensure that a PDF will display correctly even if a
font is no longer available. It prohibits features like
transparency because, at the point that the standard was created,
many tools did not support transparency.
PDF/A Part 1 Level A conformance is essentially the same as
Level B but it also requires features like Unicode text and a
tagged PDF structure.
PDF/A Part 2 Level B is the base standard. It is essentially an
updated version of Part 1 Level B, allowing features like
transparency which had been prohibited in the original
PDF/A Part 2 Level U is essentially the same as Level B but it
mandates that all text must be held in Unicode format.
PDF/A Part 2 Level A is essentially the same as Level U but
includes some elements related to tagged PDF. Many of these are
semantic rather than structural and so it is difficult to validate
in an automated system. For example it specifies that images of
text should be tagged with the text seen in the image.
PDF/A Part 3 included new features to allow other file formats
to be represented in a PDF/A compliant package. So it defines how
one should create a compliant PDF in which the original document -
for example a Word document - is embedded. In this way you have
both a document which will appear in the same way as the original
document but also an original file, in case that is required.
PDF/A Part 4 enhances the standard for use in conjunction with