Introduction - PDF Explained - page 3

Random access and linearization
Unlike PostScript, any object (page, graphic etc.) in a PDF document can be accessed
at will, in constant time. This means it's no harder to read page 150 than page 1.
Linearization is the process of arranging the objects in the file such that all those needed
for a given page are located in adjacent positions. This explains why you can quickly
jump to any page in a PDF being viewed in Acrobat Reader in a web browser window
—the viewer doesn't need to load the whole file to begin with, it fetches from the server
just the sections needed to display each new page.
Stream creation and incremental update
Stream creation is the ability inherent in the PDF format to allow files to be created
in order, from beginning to end, even if the eventual file is larger than the memory
available.
Incremental update means that, when editing a file, it's possible to write the changes
to the end of the file without modifying any existing part—this makes saving changed
versions very fast, and can be used to provide an undo mechanism (since the previous
version is still intact).
Embedded fonts
Fonts used in a PDF are embedded along with the document. This means that it should
always be rendered correctly, regardless of which fonts are installed on a given com-
puter. The program creating the PDF document will remove unnecessary data from the
font (such as metrics and unused characters), so the file does not become unduly large.
PDF supports all common font formats, such as TrueType and Type 1.
Searchable text
Most PDF files maintain the information to map the character shapes making up the
text to Unicode character codes. This means that you can copy and paste text from a
document, or search the text easily. More recent developments in PDF allow the logical
order of the text in the document to be stored separately from the layout of the text on
the page, preserving yet more structured information.
ISO Standardization
PDF was released as an open standard by the International Organization for Standard-
ization (ISO) in 2008. The ISO-32000-1:2008 document is largely the same as the PDF
file format document previously released by Adobe.
This independence lends legitimacy and oversight to the PDF standard, which should
encourage its further adoption. However, with no real tools for detecting whether a file
meets the standard (Adobe Reader will happily load malformed files, so many tools
create them), genuine rigor is some time away.