Slim down your PDFs

Adam Engst
6 November, 2008
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You can never be too rich or too thin, the old saying goes, but when it comes to Adobe’s Portable Document Format (PDF), adding richness in the form of images and fonts causes otherwise svelte PDFs to start pining for elastic waistbands. Balancing quality and file size in PDFs can be tricky, but in my five years of publishing PDF books I’ve learned—through much trial and error—numerous tricks that can help you keep your PDFs small, eliminating problems with bounced e-mail attachments, unnecessarily long downloads, and higher-than-necessary bandwidth bills.

Slimming Strategies. PDFs balloon to tens of megabytes for three main reasons: images, fonts, and the PDF format itself. Images cause the most trouble, since many programs (Keynote, for example) embed them at much higher resolutions than you’d ever actually display. By reducing those images’ size and compressing them, you can reclaim a lot of space. And if your PDF uses the same image repeatedly, Mac OS X’s built-in PDF engine stores each instance separately, instead of reusing a single version of the image.

Fonts can add bulk, too, but if you’ve used standard fonts, there’s little need to embed them in the file (as Mac OS X does by default). If you’re using nonstandard fonts, you can embed only the characters you use, rather than the entire font.

And if you do a lot of editing and page replacement in Preview or Adobe Acrobat Pro, the size of your file will grow every time you save, requiring the use of Acrobat Pro’s Save As command to reclaim the wasted space.

How you go about slimming down your PDFs depends largely on how much you want to spend on tools, and how much control you want to exert over the process. If dealing with PDFs is only an occasional experience for you, Leopard’s Preview utility may offer all the compression capabilities you need.

For more control, a utility like Apago’s $US35 PDF Shrink provides additional capabilities without breaking the bank. And if creating reasonably sized PDF files for public distribution is core to your business, as it is to mine, you need Adobe’s $679 Acrobat Pro, likely along with a tool like PDF Shrink’s big brother, the $US199 ($A294) PDF Enhancer (also from Apago). [Ed. -- we've also had some luck with Skycoast Pictures' free Compress PDF Workflow 4.0 tool, which adds Automator actions to the standard Print/PDF menu]

Preview and ColorSync. Though few people realise this, you can reduce the size of PDF files using the Leopard version of Preview. To shrink a PDF file, open it in Preview, choose Save As from the File menu, and, in the Save dialog box, choose Reduce File Size from the Quartz Filter pop-up menu. If when you compare the compressed PDF with your original, the images are too fuzzy for your needs (the default settings are pretty severe), you can make your own Quartz filter with different settings.

ColorSync utility

Duplicate the Reduce File Size filter to create a custom one with different image sampling and compression settings.

To do this, launch ColorSync Utility (in Applications/Utilities), choose New Utility Window from the File menu if none is showing, and click on Filters in the toolbar. Click on the arrow to the right of Reduce File Size, choose Duplicate Filter from the drop-down menu, and name your new filter. Enter different values for Image Sampling and Image Compression, switch back to your original PDF in Preview, and save a PDF with your new filter in place of Reduce File Size. With some trial and error, you should be able to arrive at a compromise that satisfies you.

PDF Shrink. PDF Shrink provides additional controls and a simpler interface than Preview. But where PDF Shrink really sets itself apart is in how it lets you create different sets of PDF compression options (one for e-mail distribution, another for printing, and so on) and in the ways you can access it.

You can drop PDF files on PDF Shrink’s Finder or Dock icon (which uses the default configuration), on the appropriate PDF configuration set in PDF Shrink’s main Document Manager window, or on an AppleScript-based droplet application associated with a particular PDF configuration set. PDF Shrink can also create PDF services that let you access specific configuration sets via the PDF drop-down menu in Mac OS X’s Print dialog box.

Acrobat’s PDF Optimizer. If you’re serious about working with PDFs, you need Acrobat Pro, which offers highly technical options for reducing PDF file size. From the Advanced menu, choose PDF Optimizer, which offers six different sets of options: compressing images, unembedding fonts, transparency, discarding objects, discarding user data, and cleaning up the file.

Although they’re all useful, pay particular attention to the image compression options, since that’s where you can usually save the most space. Also in the PDF Optimizer dialog box is an Audit Space Usage button, which displays another dialog box showing you what percentage of your file different PDF components occupy.

You’ll realize the majority of space savings in compressing your images, but be careful not to degrade image quality too much. The compressed version here (right) has lost some detail and sharpness, especially in the boy’s face.

Work carefully with Acrobat Pro’s PDF Optimizer. Many of the techniques it uses will degrade image quality and remove useful PDF features such as links and bookmarks. Also, when you click on OK in the PDF Optimizer dialog box, Acrobat Pro prompts you for a new file name and makes your changes, leaving the modified file open when you’re done. If you’re testing different settings, make sure to close that modified file and return to your original before testing new compression settings. Otherwise, your file could end up doubly compressed, confusing your test results and rendering the images unreadable.

When you think you’ve arrived at your ideal settings in the PDF Optimizer dialog box, save them as a custom set in Acrobat Pro, apply them to your original file, and then choose Save As from the File menu. Despite all the work you’ve been doing in the PDF Optimizer, a simple Save As can sometimes save even more space, and better yet, all of its space savings are lossless when you use the Reduce File Size filter.

If you want the control offered by Acrobat Pro but need more automation options, Apago’s PDF Enhancer provides nearly all of Acrobat’s optimisation controls along with additional PDF manipulation prowess, such as the ability to change page size, set view options, apply layouts such as booklets, stamp files with graphics, and set metadata. For my e-books, I always do a Save As in Acrobat Pro, after which I drop the e-book file on a customised configuration in PDF Enhancer for the remaining compression work.

Wrapped Up Tightly. Despite the ever-increasing availability of high-bandwidth Internet connections, file size still matters when you’re sending e-mail attachments or when you have recipients downloading your files over slow connections. If you’re distributing PDF files via the Internet, it takes only a few minutes to compress the file via Preview, a utility like PDF Shrink or PDF Enhancer, or Adobe’s full-fledged Acrobat Pro. Take the time to compress your files up front, and you’ll reap the rewards of fewer bounced e-mail attachments, faster downloads, and lower bandwidth bills.

[Adam Engst is the publisher of TidBits and the Take Control series of PDF books. The need to reduce the file size of his e-books has forced him to learn more about PDF compression than he ever wanted to know.]

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