Image downsampling and compression is another way to optimize your PDF file by reducing its final size.
Downsampling is a procedure through which a raster image resolution is reduced; and compression is when an algorithm (a mathematical formula) is applied to an image to compress it and reduce its size. Acrobat supports both.
The first time to use downsampling and compression in a PDF is when you create it, and the Images panel in the Adobe PDF Settings dialog is there to help you. Let's say you have a product sell sheet created for printing using InDesign, with some text and a few product photos at a resolution suitable for print. Now let's say you also want the sell sheet in a PDF file that you can easily email to some clients out of town. You can print the file to the Adobe PDF driver and use the Images panel to compress the images right there. The settings are very clear, offering you several compression methods (lossless ZIP or lossy JPEG) for different types of images and the ability to set downsampling parameters for each of these types of images (see Figure 42.4). For repeating jobs you can save those settings.
Figure 42.4. The Images panel in the Adobe PDF Settings dialog enables great customization.
What if all you have is the PDF file and not the original document? You can apply the same downsampling and compression parameters from within Acrobat by opening the PDF Optimizer and navigating to the Images panel. After accepting your desired settings simply save your file.