About compression and resampling in Acrobat 9

Learn about choosing the right compression method and resampling bitmap images in your file to reduce the file size.

By Adobe – July 20, 2009

 

Many factors affect file size and file quality, but when you're working with image-intensive files, compression and resampling are important.

You can choose from a variety of file compression methods designed to reduce the file space used by color, grayscale, and monochrome images in your documents. Which method you choose depends on the kind of images you are compressing. The default Adobe PDF presets use automatic (JPEG) compression for color and grayscale images and CCITT Group 4 compression for monochrome images.

In addition to choosing a compression method, you can resample bitmap images in your file to reduce the file size. A bitmap image consists of digital units called pixels, whose total number determines the file size. When you resample a bitmap image, the information represented by several pixels in the image is combined to make a single larger pixel. This process is also called downsampling because it reduces the number of pixels in the image. (When you downsample or decrease the number of pixels, information is deleted from the image.)

Neither compression nor resampling affects the quality of text or line art.