PDF Optimizer

Monochrome

downsample Ability to specify a target resolution and a trigger resolution at which monochrome images will be recompressed.
trigger-dpi All monochrome images above this resolution will be downsampled.
target-dpi The new resolution of downsampled monochrome images.
recompress Sets the type and quality of compression used to downsample monochrome images.

JBIG2 is a compression algorithm designed for binary images, or images where each pixel can only have one of two possible colors. For It can be used for either lossy or lossless image processing.

CCITT Group 4 refers to the compression type from the International Telegraph and Telephone Consultative Committee (CCITT),or TIU. Many fax and document imaging file formats support this form of lossless data compression encoding. These protocols are referred to as CCITT Group 3 and Group 4 compression, respectively.

Lossy and lossless refer to the approach used for compressing data. For lossless, all of the data in the image is preserved. The quality of the image does not change, and it can be uncompressed to its original state. Lossy compression permanently removes data from the image file, such as pixels, reducing the image resolution. Files reduced using lossy compression will be considerably smaller, but will not print or display as well as those compressed using lossless compression.

type same Keep original default compression algorithm provided in the images themselves
jbig2 Use jbig2 compression
ccittg3 Use ccittg3 compression
ccittg4 Use ccittg4 compression
quality lossy Valid for jbig2 only
lossless Valid for jbig2 only

Optional. Enable any of these values by adding it to the profile and setting it equal to ON.

optimize-images-only-if-reduction-in-size Set this value to ON if you want PDF Optimizer to only downsample an image found in a PDF document if the newly downsampled image is in fact smaller than the original. When recompressing an image, the output file that results can possibly expand in size.  If the process yields an image that is the same size as the original, or larger, PDF Optimizer will leave the image alone.
consolidate-duplicate-image-and-forms Remove duplicate copies of alternate images and forms.
down-convert-16-to-8-bpc-images When enabled, images that are 16 bits per component will be converted to 8 bits per component.

The color depth of an image is the number of bits used per pixel for each color component. RGB, for example, has three color components. By down-converting an image in a PDF file from 16 bpc to 8 bpc, you are reducing the resolution of the image, but also significantly reducing its size. If a PDF document features high-resolution images, the final PDF can also be significantly smaller.

This feature is not applicable if Color Conversion is enabled (see Color Conversion). Color Conversion will attempt to down-convert 16 bpc images automatically if you turn it on. The down-convert feature only has an impact if Color Conversion is turned off.