When you save an Illustrator file as a PDF file, 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 contains three sections. Each section provides the following options for compressing and resampling color, grayscale, or monochrome images.
Compression (Downsampling): If you plan to use the PDF file on the web, then use downsampling to allow for higher compression. If you plan to print the PDF file at high resolution, then don't use downsampling. Deselect the option to disable all downsampling options.
Downsampling refers to decreasing the number of pixels in an image. To downsample color, grayscale, or monochrome images, choose an interpolation method, enter the desired resolution (in pixels per inch). Then, enter a resolution in the For Images Above box. Illustrator downsamples all images with resolution above this threshold.
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.
- 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.
- 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.
Compression (Compression): Determines the type of compression used. The Automatic option automatically sets the best possible compression and quality for the artwork contained in the file. For most files, this option produces satisfactory results. Use Automatic (JPEG) if you need the greatest compatibility. Use Automatic (JPEG2000) for superior compression.
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 can be lossless or lossy, depending on the Quality setting.
JPEG compression is suitable for grayscale or color images. JPEG compression is lossy, which means that it removes image data and can 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.
CCITT and Run Length compression 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.
Compression (Image Quality): Determines the amount of compression that is applied. The available options depend on the compression method. For JPEG compression, Illustrator provides Minimum, Low, Medium, High, and Maximum Quality options. For ZIP compression, Illustrator provides 4-bit and 8-bit Quality options. If you use 4-bit ZIP compression with 4-bit images, or 8-bit ZIP compression with 4-bit or 8-bit images, the ZIP method is lossless. That is, data is not removed to reduce file size, so image quality is not affected. Using 4-bit ZIP compression with 8-bit data can affect the quality, however, because data is lost.
Tile Size option: The Title Size option is only enabled when the compression setting is JPEG2000. This option determines the size of the tiles for progressive display.
Compress Text And Line Art option: The Compress Text And Line Art option applies ZIP compression to all text and line art in the file. This option results in no loss of detail or quality.
Fonts option: The Subset Embedded Fonts When Percent Of Characters Used Is Less Than [x]option specifies when to embed the entire font (as opposed to just the characters used in the document) based on how many of the font's characters are used in the document. For example, if a font contains 1,000 characters but the document only uses 10 of those characters, you could decide that embedding the font isn't worth the extra file size.