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Lesson 4. Customizing Adobe PDF Output Q... > Using custom compression settings

Using custom compression settings

The default Distiller job options are designed to produce optimum results in most cases. However, you can set job options manually if you want to fine-tune the compression methods used by Distiller. In this part of the lesson, you'll practice applying custom compression and resampling settings to the color PostScript file.


Compression and resampling do not affect the quality of either text or line art.

Changing the Distiller job options

By combining the appropriate compression and downsampling job options, you can greatly reduce the file size of a PDF document without losing noticeable detail in an image. You'll apply your custom settings to the original high-resolution PostScript file Color.ps.

In Acrobat, choose Tools > Distiller.

In the Acrobat Distiller dialog box, choose Screen from the Job Options pop-up menu.

Choose Settings > Job Options, and click the Compression tab.

The default compression values associated with the Screen settings are displayed. You'll now adjust several options to produce your own custom setting (using the Screen settings as your base) for optimizing on-screen PDF display.

In the Color Images area, choose Average Downsampling To from the pop-up menu.

Average downsampling averages the pixels in a sample area and replaces the entire area with the average pixel color at the specified resolution. A slower but more precise approach is to use bicubic downsampling, which uses a weighted average to determine pixel color. Bicubic downsampling yields the smoothest tonal gradations. Subsampling, which chooses a pixel in the center of the sample area and replaces the entire area with that pixel at the specified resolution, significantly reduces the conversion time but results in images that are less smooth and continuous.

Enter 108 dpi for the resampling value and for the Images Above option.

This will downsample the original PostScript color image file (assuming the image resolution is above 108 dpi) to a resolution of 108 dpi. Options you enter in the Color Images section of the dialog box affect only color images. Any changes you make to the grayscale or monochrome options have no effect on color images. Distiller recognizes the type of PostScript image file, and applies the appropriate color, grayscale, or monochrome compression settings.


The minimum resolution of images that will be downsampled is determined by the resampling value you choose.

Leave the Compression set to Automatic to allow Acrobat to determine the best compression method, and leave the Quality set to Medium.

With the Automatic option, Acrobat determines the best compression method and quality for your color or grayscale images: JPEG is applied to 8-bit grayscale images and to 8-bit, 16-bit, and 24-bit color images when the images have continuous, smooth tones; ZIP is applied to 2-bit, 4-bit, and 8-bit grayscale images, to 4-bit color images and indexed 8-bit color images, and to 16-bit and 24-bit color images when the images have sharp color changes.

Since the Color.ps file you'll be converting contains no grayscale or monochrome images, you'll leave the default values in these sections.

If the Optimize for Fast Web View option is selected in the General tab, text and line art are compressed regardless of whether you select the Compress Text and Line Art option in the Compression tab.

Now you'll save the custom setting that you have specified so that you can use it again in the future.

Click Save As. Save the custom setting using the default name Screen(1).joboptions. You cannot overwrite the default Distiller job options.

Your custom setting will now be available from the Distiller Job Options menu, along with the default settings.

Click OK to exit the job options. Leave the Distiller window open.

About methods of compression

Distiller applies ZIP compression to text and line art; ZIP or JPEG compression to color and grayscale bitmap images; and ZIP, CCITT Group 3 or 4, or Run Length compression to monochrome images.

  • ZIP is a compression method that works well on images with large areas of single colors or repeating patterns, such as screen shots and simple images created with paint programs, and for black-and-white images that contain repeating patterns. Acrobat provides 4-bit and 8-bit ZIP compression options. If you use 4-bit ZIP compression with 4-bit images, or 8-bit ZIP with 4-bit or 8-bit images, the ZIP method is lossless, which means it does not remove data to reduce file size and so does not affect an image's quality. However, using 4-bit ZIP compression with 8-bit data can affect the quality, since data is lost.


    Adobe's implementation of the ZIP filter is derived from the zlib package of Jeanloup Gailly and Mark Adler, whose generous assistance we gratefully acknowledge.

  • The JPEG (Joint Photographic Experts Group) compression method is suitable for grayscale or color images, such as continuous-tone photographs that contain more detail than can be reproduced on-screen or in print. JPEG is lossy, which means that it removes image data and may reduce image quality, but it attempts to reduce file size with the minimum loss of information. Because JPEG eliminates data, it can achieve much smaller file sizes than ZIP compression.

    Acrobat provides five JPEG options, ranging from Maximum quality (the least compression and the smallest loss of data) to Minimum quality (the most compression and the greatest loss of data). The loss of detail that results from the Maximum and High quality settings are so slight that most people cannot tell an image has been compressed; at Minimum and Low, however, the image may become blocky and acquire a mosaic look. The Medium quality setting usually strikes the best balance in creating a compact file while still maintaining enough information to produce high-quality images.

  • The CCITT (International Coordinating Committee for Telephony and Telegraphy) compression method is appropriate for black-and-white images made by paint programs and any images scanned with an image depth of 1 bit. CCITT is a lossless method.

    Acrobat provides the CCITT Group 3 and Group 4 compression options. CCITT Group 4 is a general-purpose method that produces good compression for most types of monochrome images. CCITT Group 3, used by most fax machines, compresses monochrome bitmaps one row at a time.

  • Run Length is a lossless compression option that produces the best results for images that contain large areas of solid white or black.

––From the Acrobat 5.0 online Help.

Processing the color file with custom settings

Now you're ready to try out your new job options setting.

In Distiller, choose File > Open. Select Color.ps in the Lesson04 folder, and click Open.

Name the Adobe PDF file Color5.pdf, and click Save to save it in the Lesson04 folder. (The default is to save the file on the desktop.)

The conversion of the PostScript file is shown in the Distiller window.

In Acrobat, choose File > Open. Select Color5.pdf and Color4.pdf (the other screen-optimized PDF file) in the Lesson04 folder, and click Open.

Choose Window > Tile > Vertically to display the files side-by-side, and view both files at 200% magnification. If needed, use the scroll bars to display the same area in each of the files.

Color5.pdf is smoother than Color4.pdf. Because Color5.pdf has a higher resolution (108 dpi rather than 72 dpi), it contains more pixel detail and finer image quality.

Color5.pdf 42KB

Color4.pdf 25KB

Choose Window > Close All to close the files without saving them.

Compare the file size of the two images. In Windows use Windows Explorer; in Mac OS, choose Get Info > General Information from the File menu. Color5.pdf is significantly larger than Color4.pdf.

Processing grayscale and monochrome images

You can experiment with applying default compression and resampling settings to a grayscale and a monochrome PostScript image file. This part of the lesson is optional.

You'll convert the sample PostScript files Gray.ps and Mono.ps, located in the Lesson04 folder to Adobe PDF using the eBook job options. (We converted the two PostScript files using the Press, Print, and Screen job options for you; however, you can experiment with creating the files yourself using these job options if you have time. You can also customize the job options settings as you did in the previous section.)

In the Distiller window, choose eBook from the Job Options menu.

The eBook setting creates output appropriate for on-screen display, such as desktop computers, laptops, and eBook reading devices.

Choose File > Open. Select Gray.ps in the Lesson04 folder on your hard drive, and click Open.

Name the Adobe PDF file Gray1.pdf, and save it in the Lesson04 folder. Distiller shows the status of the conversion process to Adobe PDF.

Repeat steps 1 through 3 for the Mono.ps file, naming the resulting Adobe PDF file Mono1.pdf.

Gray1.pdf 94KB

Gray2.pdf 748 KB

Gray3.pdf 357KB

Gray4.pdf 31KB

Mono1.pdf 8KB

Mono2.pdf 8KB

Mono3.pdf 388KB

Mono4.pdf 4KB

In Acrobat, open the “Gray” series of files and the “Mono” series of files, and compare the image quality and file size as you did for the Color1.pdf, Color2.pdf, Color3.pdf, and Color4.pdf files in “Using default compression settings” on page 82.

You'll notice that images with larger file sizes do not necessarily yield better display quality. Downsampling a monochrome file may not significantly reduce its size. When converting your images to Adobe PDF, choose compression and resampling options that will give you adequate quality at the smallest file size possible.

When you've finished comparing image quality and file size, choose Window > Close All to close all the files without saving them.

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