When distributing a PDF online, some vector drawings outweigh their usefulness.
Vector drawings yield the highest possible quality across all media. For simple illustrations such as charts and graphs, they are also more efficient than bitmaps. However, when preparing a PDF for online distribution, you will sometimes find an intricate vector drawing that has tripled your PDF's file size. With Acrobat and Illustrator (or Photoshop), you can rasterize this detailed drawing in-place and reduce your PDF's file size.
How does this happen? Vector artwork scales easily without altering its quality. This means a big, detailed, 2 MB vector drawing can be scaled down perfectly to the size of a postage stamp. Even though most of its detail might no longer be visible on a paper printout or on-screen, the drawing is still 2MB in size. Again, this becomes an issue only when you go to distribute this file online and you want to reduce the document's file size.
If you have Adobe Acrobat 6 Pro or Acrobat 5 and Adobe Illustrator or Adobe Photoshop, you can rasterize a PDF's drawings. First you must configure Acrobat's TouchUp Object tool to open your PDF selections in Illustrator or Photoshop.
In Acrobat, select Edit Preferences General . . . TouchUp. Click Choose Page/Object Editor and then browse over to Illustrator.exe, which might be located somewhere such as C:\Program Files\Adobe\Illustrator 9.0.1\. Or, use Photoshop instead of Illustrator by browsing over to Photoshp.exe, which might be located somewhere such as C:\Program Files\Adobe\Photoshop 6.0\. Click Open and then click OK to confirm your new Preferences setting.
First, make a backup copy of your PDF so that you can go back to where you started at any time.
Open your PDF in Acrobat and locate the drawing you want to rasterize. Activate the TouchUp Object tool (Tools Advanced Editing TouchUp Object, in Acrobat 6) and try to select the drawing. This usually requires patience and experimentation because one illustration might use dozens of separate drawing objects. And, it usually is tangled with other items on the page that you don't want to rasterize.
First, try dragging out a selection rectangle that encloses the artwork. If other, unwanted items get caught in your dragnet, try dropping them from your selection by holding down the Shift key and clicking them. If you missed items that you wanted to select, you can add them the same way: Shift-click. The Shift key is a useful way to incrementally add or remove items from your current selection. You can even hold down the Shift key while dragging out a selection rectangle. Items in the rectangle will be toggled in or out of the current selection, depending on their previous state.
If you accidentally move an item, immediately press Ctrl-Z (Edit Undo) to restore it. If things ever get messed up, close the PDF without saving it and reopen it to start again.
After your selection is made in Acrobat, right-click inside the selection and click Edit Objects . . . . Adobe Illustrator will open and your selected material will appear. Now, you must select the items you want to rasterize. If your selection in Acrobat worked just right, you can simply select the entire page (Edit Select All). If your original selection included unwanted items, carefully omit these items from this new selection. The Shift key works the same way in Illustrator as it did in Acrobat, as you assemble your selection.
After making your selection in Illustrator, select Object Rasterize . . . and a dialog opens. Select a suitable color model (e.g., RGB) and resolution (e.g., 300 pixels per inch) and click OK. Inspect the results to make sure the artwork retained adequate detail. If you like the results, save and close the Illustrator file. Acrobat will automatically update the PDF to reflect your changes. If you still like them, save the PDF in Acrobat. Otherwise, discard them by pressing Ctrl-Z (Edit Undo) or by closing the PDF and starting over.
If you are using Photoshop instead of Illustrator, you won't have a chance to select the objects you want rasterized; Photoshop immediately rasterizes everything you selected in Acrobat. One advantage of using Photoshop is that it won't try to substitute fonts, as Illustrator sometimes does.
After your selection is made in Acrobat, right-click inside the selection and click Edit Objects . . . . Photoshop will open and ask you for a resolution. Enter a resolution (e.g., 300 pixels per inch), click OK, and the rasterized results will appear. Inspect the results to make sure the artwork retained adequate detail. If you like the results, save the Photoshop file. By default, it should save as PDF, but sometimes you must change the Format to PDF in the Save dialog. Before saving the file, Photoshop will ask which encoding to use (ZIP or JPEG). If you choose JPEG, you can also set its quality level. After saving the rasterized artwork in Photoshop, Acrobat will automatically update the PDF to reflect your changes. If you still like them, save the PDF in Acrobat. Otherwise, discard them by pressing Ctrl-Z (Edit Undo) or by closing the PDF in Acrobat and starting over.
Sometimes, when the rasterized artwork is brought back into the PDF, it will cover up and obscure other items on the page. The trick is to place the new bitmap behind the obscured items.
In Acrobat 5, select the bitmap with the TouchUp Object tool, right-click, and select Cut. Right-click the page anywhere and select Paste In Back. The bitmap should appear in the same location, but behind the other items on the page. If it didn't reappear, it is probably being obscured by a larger, background item. Select this obscuring object and cut-and-paste it the same way. Save the PDF when you are done.
In Acrobat 6, open the Content tab (View Navigation Tabs Content) and click on the plus symbol to open a hierarchy of document page references. Locate your page and then click on its plus symbol to open a hierarchy of page objects. Lower objects on this stack overlap the higher objects. Identify your image (it will be wrapped inside an XObject node), then click and drag it to a higher level in the page hierarchy. This will take some experimentation. See [Hack #63] for tips on dragging and dropping these nodes. Save the PDF when you are done.