When saving artwork in 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 Adobe PDF Options dialog box is divided into three sections. Each section provides the following options for compressing and resampling 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, do not use downsampling. Select the Do Not Downsample option to disable all downsampling options.

Downsampling refers to decreasing the number of pixels in an image. To downsample images, choose an interpolation method—average downsampling, subsampling, or bicubic downsampling—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 are downsampled.

The interpolation method you choose determines how pixels are deleted:

Average Downsampling

Averages the pixels in a sample area and replaces the entire area with the average pixel color at the specified resolution. Average downsampling is the same as Bilinear resampling.

Subsampling

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. Subsampling is the same as Nearest Neighbor resampling.

Bicubic Downsampling

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 gradations.

Compression

Determines the type of compression that is used.

ZIP compression

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 is lossless.

JPEG compression

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 file sizes than ZIP compression.

JPEG2000

Is the new international standard for the compression and packaging of image data. Like JPEG compression, JPEG2000 compression is suitable for grayscale or color images. It also provides additional advantages, such as progressive display and lossless compression not available with JPEG. JPEG2000 is only available if Acrobat 6 (PDF 1.5) or later is selected from the Compatibility menu.

Image Quality

Determines the amount of compression that is applied. The available options depend on the compression method. For JPEG2000 compression, Photoshop provides Lossless, Maximum, High, Medium, Low, and Minimum options. For JPEG compression, Photoshop provides Minimum, Low, Medium, High, and Maximum options. For ZIP compression, Photoshop provides an 8‑bit Image Quality option. The 8‑bit Image Quality option is lossless; that is, data is not removed to reduce file size, so image quality is not affected.

Tile Size

Specifies the size of the tiles used in images with JPEG 2000 compression. When low Image Quality values are used to optimize images smaller than 1024 x 1024 pixels, using the largest tile size produces better results. In general, a tile size of 1024 is best for most images. Lower tile sizes are generally used for images with small dimensions (for viewing on devices such as cell phones).

Convert 16 Bit/Channel Image To 8 Bit/Channel

Converts 16‑bits-per-channel images to 8‑bits-per-channel images (selected by default). ZIP is the only compression method available if the Convert 16 Bits option is unselected. If your document’s Compatibility setting is Acrobat 5 (PDF 1.4) or earlier, the Convert 16 Bits option is unavailable, and images are automatically converted to 8 bits per channel.