COMPRESSING PDF FILES

To reduce file size , Acrobat gives you the option to compress contained images using either ZIP or JPEG compression algorithms. Without getting too technical, the difference between these two is that ZIP compression maintains an image's quality integrity. This type of compression is sometimes referred as lossless . JPEG compression degrades the image quality and is referred to as lossy . JPEG compression achieves smaller files than ZIP but not without sacrificing quality. Acrobat also enables you to choose None, leaving your embedded images uncompressed. This option makes your PDF files larger.

As a general rule you should try to avoid JPEG compression, especially if you are working on documents intended for print. If your document is to be displayed on screen only you may get away with a limited amount of JPEG compression without any visible loss of quality.

The reason you should not compress your PDF images using JPEG compression is that the JPEG algorithm compresses images even if they've already been compressed. Because JPEG is a lossy algorithm, this means that every time you save your file the embedded images are being recompressed and more data is being lost.

Now, here's the tricky part: If you plan to use JPEG compression, do not compress your images until you have the final version of your document ready. This means that when you create your PDF you shouldn't apply any type of compression. To do this, open the Compression panel in the Save Adobe PDF dialog when you save your file from its originating application to PDF. While in the Compression panel, select Do Not Downsample for all types of images and also disable the Compress Text and Line Art check box (see Figure 44.5). This produces a larger PDF file, but you will have the chance to compress it later when you finish editing it in Acrobat.

Figure 44.5. If you plan to edit your PDF in Acrobat do not apply any compression to it when creating it.

After you have the final version of your document you can use the PDF Optimizer (described in Chapter 42) to apply compression to your images.

PREPARING FILES FOR PRESS

If your intended output is a printing press, Acrobat offers a wide array of tools to make sure your final files print the way they should. These tools are grouped in the Print Production toolbar. Let's take a look at what each of these can do for you.

Trap Presets

Because of the way inks are applied when printing color in an offset preset and because of the elasticity of the paper stock used for print, sometimes a thin gap may appear between two areas of different colors. To avoid that from happening traps are applied. Traps are very thin lines of overlapping inks applied to those critical areas to prevent these gaps from appearing. To create a trap, normally the area of the lightest color is slightly expanded to fit under the area of darker color. Acrobat files can contain specific information that enables printers to generate traps automatically when sending a job to press. The default Trap Presets contain generic values for the automatic creation of traps for most cases, but sometimes a job requires these values to be changed. Acrobat gives you the flexibility not only to change these default values but also to save them as a recorded preset. If you have doubts about setting traps, consult with your printer or service bureau . Most printers prefer to do the trapping themselves , so most of the time you should not worry about trapping. However, if you are producing your own color separation films , these trap settings are available to you. Settings may vary depending on the particular characteristics of the paper and inks to be used (see Figure 44.6). The available settings are

Trap Width : This value determines the width the created trap lines should have in points (points are a measuring unit used in the graphics industry. One point is roughly equivalent to 1/72 of an inch). A trap too wide becomes too visible to the naked eye and makes the image look bad. The value for the Default option applies to all inks except black. The Black option has its own separate value and we'll explain why later in the chapter. The value for the Images option is for all images if you want to use different trapping widths for images and for colored text or other objects.
Trap Appearance : This refers to the way traps behave around corners or on their edges. Setting the Join Style option to Miter makes the traps have angled corners and edges, and setting the option to Round makes them look rounded. The End Style option determines the way traps are shaped when more than two of them meet.
Images : These options determine how traps behave between or within images. The Trap Placement option refers to how traps are created between vector objects and raster images. Enable the Trap Objects to Images option to ensure that traps are created when vectors and rasters meet. If this option is unchecked no trap will be created. The Trap Images to Images option does the same when two rasters meet. The Trap Images Internally option has Acrobat generate traps for color areas within a raster image and not only around its edges. The Trap 1-bit Images option tells Acrobat to create traps for that type of image as well.
Trap Thresholds : These values refer to the conditions under which the traps are generated. The Step value is the amount of displacement between colors that must occur before a trap is generated. The lower this number the more sensitive Acrobat will be to variations. The Black Color value specifies how much black ink there must be before a black trap is generated. The Black Density value applies to spot inks and tells Acrobat when to apply the black ink settings to it. The default setting of 1.6 means that any spot ink with a neutral density of 1.6 or higher should be given the same trapping treatment as if it were black. Consult your printing service if you need to obtain neutral density values for special inks you plan to use. The Sliding Trap value defines when Acrobat should generate a dynamic trap that shifts position depending on the variation of density between two color areas. If you are using pastel inks, for example, you may need to give them a special treatment to avoid having the trapping area become darker than the actual color area. The Trap Color Reduction value helps you do this. A 100% value produces a trap using 100% of the inks that form it, and a 0% value reduces the amount of ink in the trapping to match the ink's neutral density. If you are not using spot pastel inks, leave this value at its 100% default.

Figure 44.6. You can customize your trapping preferences and save them for future use.

What Is Neutral Density?

Conversion of spot colors to the CMYK space is never exact, and most of the time all you get is a close approximation . This difference becomes even more obvious if you consider special spot inks like varnishes, neons, pastels, or metallic hues. An ink's neutral density is a value that tells you how much lighter or darker a spot ink is than its CMYK equivalent. This value tells a commercial printer which of two or three spot inks "looks" the darkest so that he can adjust the trapping order. A metallic ink, for example, has a higher density than a process black ink so it should be applied after the black ink and not before. When working with two adjacent metallic inks, a printer can look at their neutral densities to determine which should be applied first.

Output Preview

The Output Preview feature enables you to see how the colors will break when producing separations and is for reference only. The preview doesn't display traps or halftone screens, and it shouldn't be used for proofing but only as way to check on the amount of ink coverage on the printed page (see Figure 44.7) or to spot any overprints quickly (see Figure 44.8).

Figure 44.7. Use Output Preview to see how each color plate will print and to check the amount of ink coverage. Too much ink can result in excessive moisture and cause paper to break when passing through a press.

Figure 44.8. Output Preview also displays overprint areas. Because inks are not entirely solid but have transparency to them, overprinting can result in big color variations in the final product.

Preflight

If you are reviewing a PDF file created by someone else, you can have Preflight generate a report in text or PDF format that you can use for reference (see Figures 44.9 and 44.10).

Figure 44.9. Preflight comes with many profiles to help you spot and correct problems before sending the file to production.

Figure 44.10. Use Preflight's reports to correct problems before files are sent to a commercial printer for production.

Convert Colors

Sometimes, due to time constraints or because the document has just too many images that need to be converted from one color mode to another, you may want to do the conversion with one click. The Convert Colors dialog is there for you. It is a no- nonsense way to straighten colors across a document quickly, especially if it's a long one (see Figure 44.11). It gives you options for three types of images: RGB, CMYK, and grayscale. You select the intended output and Acrobat converts the images. Keep in mind that this tool does not magically retouch and correct color. If an embedded image is of poor quality there's nothing that Acrobat can do about it.

Figure 44.11. The Convert Colors dialog is a quick way to change color modes to images all across a document or on selected pages.

Because the cost of reproducing a job increases with the number of inks required, you can also use the Ink Manager to reduce the amount of inks by mapping an ink to another. There are times, for example, when an artist may have chosen two slightly different spot colors when only one was needed. By mapping one of them to the other in the Convert Colors dialog (using the Action pop-up menu), you tell Acrobat to create one separation plate for both rather than two individual plates (see Figure 44.12). After color has been mapped to another, it can't be reversed to its original.

Figure 44.12. Use the Ink Manager to change ink density, trapping order of inks, or to alias one ink to another.

Ink Manager

The Ink Manager is a fast way to preview how many inks are going to be required to reproduce a job. This is of special importance when working with spot colors since the cost of reproducing a job increases with the number of inks required. If you need to reduce the amount of inks used in a job but don't want to map similar inks, you can use the Ink Manager to alias one ink to another (see Figure 44.12). Aliasing inks produces the same result as mapping but, unlike mapping, it doesn't alter the images in the PDF.

The Ink Manager also enables you to change the trapping sequence of inks (the order in which inks are printed in a press). Because inks are not entirely opaque but have varying degrees of transparency, the order in which they are printed can make a difference to the final look of your printed document, especially in trapped areas.

Add Printer Marks

Printer marks are markings placed on film, outside the actual print area, that help printers by identifying what plate should be what color, the positioning of the page, the name of the job, registration marks, and so forth. Without printer marks a printer would be clueless as to what to do with a piece of film after it's produced. Acrobat can generate these printer marks automatically based on parameters you feed it in the Add Printer Marks dialog (see Figure 44.13).

Figure 44.13. The Add Printer Marks dialog.

Crop Pages

What happens if the page size in Acrobat is not the final page size intended? A page in Acrobat could contain bleed areas or be a two-page spread, for example, in which case autogenerated printer marks would be placed in the wrong places. In situations like that, the Crop Pages dialog enables you to set up margins to define the exact size the finished product should be.

The crop box is the area that the final product will be and it is the area that the printer marks will frame. The trim box represents the actual size of your PDF page. The bleed box is the area that will be printed outside the trim box when there are bleeding objects or images (a bleed is when a page is printed all the way out to its edge). The art box (or media box ) is the size of the page as it was exported from the application that created it (see Figure 44.14).

Figure 44.14. The boxes in the Crop Pages dialog.

Fix Hairlines

Fix Hairlines is a straightforward feature. If there are any thin lines in the PDF that are part of text strokes or part of a vector object, Acrobat makes sure that they're not too thin and bumps up their width so that they show when the document is printed. Acrobat uses the parameters you supply in the Fix Hairlines dialog to accomplish this.

Transparency Flattening

If your document contains overlapping raster images with transparency (like embedded GIF or PNG images), Acrobat flattens them at print time. However, if your document contains overlapping transparency vectors, Acrobat breaks each transparent area into pieces of solid (flat) color that can be either new vectors or rasters. You can determine how much of these transparent areas are converted to rasters or vectors using the Transparency Flattening feature and its Raster/Vector Balance slider. The lowest setting produces all the areas to become rasters, and the highest setting makes them all vectors. You can also determine the resolution of these generated rasters.

The vector/raster conversion is important if you are concerned about file size. Converting all transparent areas to raster produces larger files, and having Acrobat generate too many small vector images adds an extra demand on processing to the RIP or printer, which can translate to a longer processing time.

PDF Optimizer

The PDF Optimizer dialog has several panels with options to clean up your PDF and make it smaller and easier to transport. Clicking on the Audit Space Usage button generates a report telling you what elements use how much space and their percentage in relation to the entire file size. After you have edited these settings you can save them for future use:

Images : In this panel you can apply compression to all rasters embedded in your file and downsample them if their resolution is too high. These settings apply across the document, not to individual images (see Figure 44.15).

Figure 44.15. If your images are already embedded at the right resolution, you can turn Downsampling off.
Scanned Pages : The settings on this panel apply compression and correction filters to images that have been acquired using the Create PDF From Scanner function. However, turning on this option disables the settings in the previous panel (see Figure 44.16).

Figure 44.16. The compression settings in the Scanned Page Settings slider override those in the Images panel.
Fonts : As explained in Chapter 43, you can use this panel to unembed fonts that are no longer required.
Transparency : The settings on this panel are the same settings found in the Transparency Flattening option.
Discard Objects : Use the settings on this panel to select types of elements to be discarded from your PDF. You should be careful using these options because discarded objects can't be brought back. The Discard Embedded Thumbnails option, for example, removes the page thumbnails you see in the Pages tab. The Discard Document Structure option removes tags and reflow information. If you have layered objects (like drawing done in Illustrator) Discard Hidden Layer Content merges all layers .
Clean Up : This panel contains complementary settings to the previous one. You can choose to apply Flate (ZIP) compression to text or other parts of the file or remove bookmarks to pages that no longer exist. The Remove Unreferenced Named Destinations option looks for destinations that have no link within the same document, so if you have links in other documents you should not check this option (see Figure 44.17).

Figure 44.17. Some of the PDF Optimizer settings are undoable and therefore you must practice caution using them.

JDF Job Definitions

Job definition files (JDFs) are small files that can be attached to a PDF and contain information, such as number of copies to be made; the type of stock to be used for cover, body, and any other special sections in your document; the person to contact; billing information; a preflight report; and more. The job definitions are saved in a file of JDF format that you can reuse in future jobs (see Figure 44.18).

Figure 44.18. Job definitions contain all the information required for a printer to reproduce your document in a press environment.

Show/Hide Print Production Toolbar

Choose Tools, Print Production to toggle between showing and hiding the Print Production toolbar.

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COMPRESSING PDF FILES