Adobe Acrobat 7 Professional - Manual (Page 508)

Using Images settings
The Images panel of the PDF Optimizer lets you set options for color, grayscale, and monochrome image
compression.
Images panel of PDF Optimizer. You can change the PDF compatibility in this dialog box.
In the Image Settings area, you can select the following options:
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Downsample reduces file size by lowering the resolution of images, which involves merging the colors of original
pixels into larger pixels. (See
Compressing and downsampling images and Images options .)
Note: Masked images and images of less than 16-by-16 pixels are not downsampled.
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Compression reduces file size by eliminating unnecessary pixel data. In general, JPEG and JPEG2000 compression
give better results on images like photographs with gradual transitions from color to color. ZIP is the better choice
for illustrations with large areas of solid, flat color or patterns made up of flat colors. For monochrome images,
JBIG2 compression, which is available in PDF Optimizer but not in Acrobat Distiller, is superior to CCITT. (See
Methods of compression .)
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Quality can be set only for JPEG and JPEG2000 compression. JPEG and JPEG2000 compression methods are
typically lossy, a process that permanently removes some pixel data. You can apply lossy JPEG or JPEG2000
compression to color images at various levels (minimum, low, medium, high, maximum). For JPEG2000
compression, you can also specify lossless so that no pixel data is removed. Compression for monochrome images
is lossless, except for JBIG2 compression, which provides both lossy and lossless modes of compression.