PDF is a digital format for representing documents. PDF files may be converted
from other electronic formats or digitized from hard copy formats. Businesses,
governments, libraries, archives and other institutions and individuals around the
world maintain considerable bodies of important information in PDF. Much of this
information must be kept for substantial lengths of time; some must be kept
permanently. These PDF files must remain useable and accessible across multiple
generations of technology. The future use of, and access to, these objects depends upon
maintaining their visual appearance as well as their higher-order properties, such as
the logical organization of pages, sections, and paragraphs, machine recoverable text
stream in natural reading order, and a variety of administrative, preservation and
descriptive metadata.