When exporting documents to Adobe PDF, you can compress text and line art, and compress and downsample bitmap images. Depending on the settings you choose, compression and downsampling can significantly reduce the size of a PDF file with little or no loss of detail and precision.

The Compression area of the Export Adobe PDF dialog box is divided into three sections. Each section provides the following options for compressing and resampling color, grayscale, or monochrome images in your artwork.

Downsampling

If you plan to use the PDF file on the web, use downsampling to allow for higher compression. If you plan to print the PDF file at high resolution, check with your prepress service provider before setting compression and downsampling options.

You should also consider whether users need to magnify a page. For example, if you are creating a PDF document of a map, consider using a higher image resolution so that users can zoom in on the map.

Downsampling refers to decreasing the number of pixels in an image. To downsample color, grayscale, or monochrome images, choose an interpolation method—average downsampling, bicubic downsampling, or subsampling—and enter the desired resolution (in pixels per inch). Then enter a resolution in the For Images Above text box. All images with resolution above this threshold will be downsampled.

The interpolation method you choose determines how pixels are deleted:

Average Downsampling To

Averages the pixels in a sample area and replaces the entire area with the average pixel color at the specified resolution.

Subsampling To

Chooses a pixel in the center of the sample area and replaces the entire area with that pixel color. Subsampling significantly reduces the conversion time compared with downsampling but results in images that are less smooth and continuous.

Bicubic Downsampling To

Uses a weighted average to determine pixel color, which usually yields better results than the simple averaging method of downsampling. Bicubic is the slowest but most precise method, resulting in the smoothest tonal gradations.

Compression

Determines the type of compression that is used:

Automatic (JPEG)

Determines automatically the best quality for color and grayscale images. For most files, this option produces satisfactory results.

JPEG

Is suitable for grayscale or color images. JPEG compression is lossy, which means that it removes image data and may reduce image quality; however, it attempts to reduce file size with a minimal loss of information. Because JPEG compression eliminates data, it can achieve much smaller files sizes than ZIP compression.

ZIP

Works well on images with large areas of single colors or repeating patterns, and for black-and-white images that contain repeating patterns. ZIP compression can be lossless or lossy, depending on the Image Quality setting.

JPEG 2000

Is the international standard for the compression and packaging of image data. Like JPEG compression, JPEG 2000 compression is suitable for grayscale or color images. It also provides additional advantages, such as progressive display. The JPEG 2000 option is only available when Compatibility is set to Acrobat 6 (PDF 1.5) or later.

Automatic (JPEG 2000)

Determines automatically the best quality for color and grayscale images. The Automatic (JPEG 2000) option is only available when Compatibility is set to Acrobat 6 (PDF 1.5) or later.

CCITT And Run Length

Are only available for monochrome bitmap images. CCITT (Consultative Committee on International Telegraphy and Telephony) compression is appropriate for black-and-white images and any images scanned with an image depth of 1 bit. Group 4 is a general-purpose method that produces good compression for most monochrome images. Group 3, used by most fax machines, compresses monochrome bitmaps one row at a time. Run Length compression produces the best results for images that contain large areas of solid black or white.

Note: Grayscale images that have been colorized in InDesign are subject to the compression settings for Color Images. However, grayscale images colorized with a spot color (and [None] applied to their frames) use the compression settings for grayscale.

Image Quality

Determines the  amount of compression that is applied. For JPEG or JPEG 2000 compression, you can choose Minimum, Low, Medium, High, or Maximum quality. For ZIP compression, only 8‑bit is available. Because InDesign uses the lossless ZIP method, data is not removed to reduce file size, so image quality is not affected.

Tile Size

Determines the size of the tiles for progressive display. This option is only available when Compatibility is set to Acrobat 6 (1.5) and later, and Compression is set to JPEG 2000.

Compress Text And Line Art

Applies Flate compression (which is similar to ZIP compression for images) to all text and line art in the document, without loss of detail or quality.

Crop Image Data To Frames

May reduce file size by exporting only image data that falls within the visible portion of the frame. Do not select this option if postprocessors might require the additional information (for repositioning or bleeding an image, for example).