Downsampling is only applied to PDF files and can greatly reduce the amount of space it takes to store the image information in your file. It does this by reducing the dots per inch (DPI) of the image. Images are made up of bits called pixels and by reducing the number of pixels in the image through downsampling you reduce the overall size of the file.
Unlike most of the Compression Preferences options, downsampling does have an effect on the quality of the images in your file. The downsampling DPI can be set any where between 50 and 600 DPI. An image will only be downsampled if its DPI is higher then the downsampling DPI. If the DPI of the image is less than the downsampling DPI the image is not changed.
Image Downsampling Options
Reduce color images to - allows you to optionally choose to downsample color images to the dots per inch selected.
Reduce indexed images to - allows you to optionally choose to downsample indexed images to the dots per inch selected.
Reduce greyscale images to - allows you to optionally choose to downsample greyscale images to the dots per inch selected.
Reduce monochrome images to - allows you to optionally choose to downsample monochrome (black and white) images to the dots per inch selected.
For all of the above options, the dots per inch can be set any where between 50 and 600 DPI by typing the desired level into the edit field, or using the spinbox up/down arrows to the right of each edit field.
