Introduction - PDF Explained - page 7

Vector Images
Graphical content in PDF is based on the model first used in Adobe's PostScript lan-
guage. It consists of paths built from straight lines and curves. Each path may be filled,
“stroked” to draw a line, or both. Lines can have varying thicknesses, join styles and
dash patterns.
Paths may be filled in any color, with a repeating pattern defined by other objects, or
with a smooth gradient between two colors. All these options apply also to the lines of
stroked paths.
Paths can be rendered using a variety of plain or gradient transparencies, with several
different blend modes defining how semitransparent objects interact. Objects may be
grouped together for the purposes of transparency, so a single transparency can be
applied to a whole group of objects at once.
Paths can be used to clip other objects, so that only sections of those objects overlapping
with the clipping path are shown. These clipping regions may be nested within one
another.
PDF has a mechanism which allows a graphic to be defined once and then used multiple
times in different contexts. This can be used, for instance, for a recurring motif, even
across more than one page.
Raster Images
PDF documents can include bitmap images between 1 and 16 bits per component, in
several color spaces (for example, three-component RGB or four-component CMYK).
Images can be compressed using a variety of lossless and lossy compression
mechanisms.
Images may be placed at any scale or rotation, used to create a fill pattern, and may
have a mask which defines how they use transparency to blend with the background
they are placed on.
Color Spaces
PDF can use color spaces related to particular electronic or print devices (grayscale,
RGB, CMYK) and ones related to human color perception. In addition, there are color
spaces for the printing industry such as spot colors . Mechanisms exist for simpler PDF
programs (like onscreen viewers) to fall back to basic color spaces if they do not support
the more advanced ones.
Metadata
PDF documents have a set of standard metadata, such as title , author , keywords and so
on. These are defined outside the graphical content and have no effect on the document