Adobe Acrobat 7 Professional - Manual (Page 163)

Compressing and downsampling images
When converting PostScript files to Adobe PDF, you can compress text and line art
(which is also called vector objects), and compress and downsample color, grayscale, and
monochrome images. Line art is described with a mathematical equation and is usually
created with a drawing program such as Adobe Illustrator. Images are described as pixels
and are created with paint programs or from scanners. Monochrome images include most
black-and-white illustrations made by paint programs and any images scanned with an
image depth of 1 bit. Adobe Photoshop, for example, works with images.
When you downsample (or decrease the number of pixels), information is deleted from
the image. With Distiller, you specify an interpolation method--average downsampling,
bicubic downsampling, or subsampling--to determine how pixels are deleted. Depending
on the settings you choose, compression and downsampling can significantly reduce the
size of a PDF file with little or no loss of detail and precision.
Related Subtopics:
Methods of compression
Applying different settings to different images