Graphics Programs Reference
In-Depth Information
Interactive Forms
There are two incompatible forms architectures in PDF: AcroForms, which is an open
standard, and the Adobe XML Forms Architecture (XFA), which is documented but
requires commercial software from Adobe.
Forms allow users to fill in text fields, and use check boxes and radio buttons. When
the data is complete, it may be saved into the document (if allowed) or submitted to a
URL for further processing. Embedded JavaScript is often used in conjunction with
forms to deal with verification of field values or similar tasks.
Logical Structure and Reflow
Logical structure facilities allow information about the structural content (chapters,
sections, figures, tables, and footnotes) to be included alongside the graphical content.
The particular elements are customizable by third parties.
A
tagged PDF
is one which has logical structure based on a set of Adobe-defined ele-
ments. Files following these conventions can be
reflowed
by a reader to display the same
text in a different page size or text size, for example in an ebook reader.
Security
PDF documents can be encrypted for security, using RC4 or AES encryption methods.
There are two passwords—the
owner password
and the
user password
. The owner
password unlocks the file for all changes, the user password just allows a range of
operations selected by the owner when the file was originally encrypted (for example,
allowing or disallowing printing or text extraction). Frequently the user password is
blank, so the file appears to open as normal, but functionality is restricted.
Starting with PDF 1.3, digital signatures can be used to authenticate the identity of a
user or the contents of the document.
Compression
Images and other data streams in PDF can be compressed using a variety of lossless and
lossy methods defined by third parties. By compressing only these streams (rather than
the whole file), the structure of the PDF objects is always available without decom-
pressing the whole file, and compressed sections can be processed only when needed.
There are several groups of compression methods:
• Lossless compression for bi-level (e.g., black and white) images. PDF supports the
standard fax encoding methods for bi-level images and, from PDF 1.4, the JBIG2
standard, which provides better compression for the same class of images.
• Lossy image filters such as JPEG and, from PDF 1.5, JPEG2000.
