Friday, September 7, 2007
by Duff Johnson
Optimize your PDFs, especially for the Web
A little knowledge can be dangerous. Few tools in Acrobat 7.0 Professional exemplify this truism more than the Optimizer (Advanced > PDF Optimizer). Due to its power, it's easy to lose or degrade precious work when the tool is used too casually. Nonetheless, the Optimizer is a fantastic tool for improving the functionality of PDF content--and it can also be run across thousands of files via Acrobat's batch mode (Advanced > Batch Processing).
This article includes an introduction to the tool and some simple rules for its use. Follow them and you'll be ready to begin improving almost any PDF file you encounter!
Introduction
What the Optimizer can do for you
- Audit documents for their relative size broken down by content types
- Reduce file size through image downsampling and recompression
- Reduce file size through the removal of extraneous content
- Enhance scanned images
- Ensure compatibility with older versions of Reader
The Optimizer was added to Acrobat Professional as a one-stop shop for final preparation of a PDF file prior to deployment. A typical use is for polishing PDF files prior to uploading to a website or for e-mail distribution. You can run the Optimizer as part of an Acrobat batch process, in which file-open options, metadata and other parameters may be set on a large number of files in a single operation.
Key features
Before looking at the Optimizer functions, spend a moment with the handy Audit Space Usage tool at the top of the Optimizer window. It's a good idea to begin with a sense of how the different elements of your PDF contribute to the overall size of the file. You'll quickly find that images usually contribute the most kilobytes, but there are other cases where extraneous data (which the Optimizer can often remove) will comprise half or more of a file's size!
The Optimizer's functions are arranged into six panels:
- Images
- Scanned Pages
- Fonts
- Transparency
- Discard Objects
- Cleanup
Each is briefly profiled below, along with some basic recommendations on their respective use.
Note that the Optimizer includes the ability to save any given configuration from all of these panels as a single preset. This is invaluable, since once you've found a given collection of settings that's right for your task, you'll want to use that configuration henceforth, or until you change your workflow.
Images
This panel allows you to manually configure the output resolution and compression settings for the images in your document. Of special note is the JBIG2 option, a vital enhancement for black-and-white scans. The best and most consistent results in terms of image resolution and compression are achieved through mastery of this panel.
Suggestions:
- Try to leave color and grayscale-image resolution at a minimum of 144 dpi. At lower resolutions, print quality becomes seriously compromised on photographic images.
- Go with Bicubic Downsampling for color and grayscale images
- Avoid downsampling monochrome images to less than 300 dpi; in general, don't downsample them at all. Use lossy JBIG2 compression instead; it's great for reducing the file size of black-and-white scans.
- Don't re-optimize with the Images panel. Each time you compress an already compressed image, you get a worse result than if you had used your compression choices on the original image.
Scanned pages
When activated by the checkbox, this panel overrides the manual settings on the Images panel in favor of some rather ineffective automation. For most users, the value of this panel lies more in the image cleanup options via the various Filters, than in the adaptive compression system itself.
Since I don't like the Adaptive Compression as offered in Acrobat 7, I recommend you use this panel only to perform image cleanup operations PRIOR to your final PDF Optimizer run, which is when you'll be deciding on image resolution and compression in the Images panel.
Suggestions:
- Use the Scanned Pages panel for image cleanup only (the Filters).
- When using this panel for image cleanup, move the Quality slider to maximum, set your desired filters and remove settings from the other Optimizer panels before saving your configuration as a preset.
- Determine your final resolution and compression via the Images panel on a separate Optimizer run.
Fonts
This panel permits the user to select individual fonts embedded in the document and unembed them to save file size. This can be of special relevance with smaller documents, where it's common to see a very high proportion of the file size (sometimes 30 percent or more) taken up by fonts. I've seen CD-ROMs of PDF files that included more than 200MB devoted to fonts alone!
As a general rule, once you've gotten to the point of using the Optimizer, it's safest to leave fonts embedded the way they are. If the specter of font substitution doesn't bother you, then by all means, unembed as you wish. You can safely unembed base fonts (Arial and Times) if they happen to be embedded.
Transparency
This is one of those panels where unless you have very specialized needs, you really don't need to know that much. Go ahead and check the Flatten Transparency box to activate this panel and accept the default settings. Don't convert text to outlines, however, unless you have a specific reason to do so!
Discard objects
In the vast majority of cases, it is perfectly safe and appropriate to check all the boxes on this panel--with the glaring exception of the Discard Document Structure item. If you enable this option, the logical structure of your file will disappear and the contents will be inaccessible to users with assistive technology. Of course, if you don't have any structure in the file or don't need an accessible file, then you can check this item.
Cleanup
In this panel, the object compression options change according to the version of Adobe's PDF specification you choose for compatibility. In general, choose compression.
Suggestions:
* All checkboxes in this panel may be safely checked for 99 percent of cases EXCEPT Remove Invalid Bookmarks and Remove Invalid Links. These settings tend to result in the loss of perfectly valid bookmarks and links in addition to invalid ones.
* One very important element of this panel is the Optimize for Fast web view option. Be sure this is checked - there are very few cases in which you'd rather not have Fast web view enabled.
Version compatibility
Basic rules for the Optimizer
- Once you find the right configuration for your needs, save it as a preset for future use.
- Use the Images panel to manage resolution and compression, use the Scanned Pages panel (if at all) in a separate Optimization operation to clean up your images using the filters.
- Default to Acrobat 5.0 compatibility (PDF 1.4 specification) unless you have a specific reason to do otherwise.
- Some things are best always left unchecked; see the notes for the Discard Objects and Clean Up panels.
A key function of the Optimizer is to ensure that your PDF is of the correct specification to support its intended contents, or else is of the lowest possible specification to ensure the best possible experience on versions of the Adobe Reader prior to 7.0.
Certain features of the PDF 1.6 specification (Acrobat 7.0) files are not available in Acrobat 5.0. Colored or boldface bookmarks, for example, are lost when converting to the lower specification. Several other features, including JPEG2000 support and certain types of security, are also unavailable in the 1.4 specification.
This matters, because only a 1.4 specification file is fully compatible with the Acrobat and Reader 5.0 programs still in wide use. Without a 1.4 specification PDF file, users with Reader or Acrobat 5.0 will get a message about out-of-date software every time they open such a file.
There are exceptions, of course. If you need or prefer to use JPEG2000 compression (there's a file-size advantage with color images), more recent versions of PDF security or fancy formatted bookmarks, you'll need to select Acrobat 6 compatibility at a minimum. But you've been warned: Many users are still using version 5.0.
Conclusion
I've prepared screenshots of the Optimizer's various panels showing our recommended basic settings for Web Content Optimization (PDF: 695 kb). Feel free to vary these to meet your needs. You'll find these are useful average settings for most PDFs intended for use as a reference document served from a website. If you need very high resolution or minimal compression due to a need to convey a high level of image detail, then by all means, alter these settings accordingly and save your new profile.
Finally, be sure that optimizing is the LAST thing you do to the file before posting it online. Any other edits or saves in Acrobat 7.0 will result in automatically upgrading the file to the 1.6 specification - Acrobat 7.0 compatibility. This means you'll have to re-optimize again just to get the compatibility back to 5.0.
Originally posted on Duff Johnson's PDF Perspective blog for acrobatusers.com.
