PDF In-Depth

Reducing PDF File Size (Optimizing PDFs)

April 07, 2014

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PDF is a mature technology that now has that standing of a de jure standard, controlled as it now is by an official standards body (the ISO). Between them, the ISO and Adobe before it have built PDF into a powerful and versatile format. It might not be capable of being all things to all people, but its flexibility is undiminished for all that.

The thing is that, while the PDF technology can now handle the inclusion of everything and the kitchen sink, it's generally not a good idea to try getting it all into a single document. The format be, well, many things to many people, but individual PDF files can't. Instead, it's better to optimize each document for its intended purpose, audience and delivery method.

"Optimizing" a PDF is often focused on reducing its file size by removing unnecessary elements. Of course, the usage case is what determines which parts are important enough to keep. In general, it is often desirable to include only crucial content elements to ensure a (relatively) lightweight file that can be easily emailed, uploaded, downloaded or whatever.

With the proliferation of high-speed internet connections and the always-online culture associated with mobile devices, optimization has become a more nuanced concept. File size is still important, especially if you want mobile users to be able to access your PDFs on-the-go. That said, if the document's purpose justifies it and your users are all working with broadband connections, why not include those high-quality graphics or fancy bells and whistles like video, dynamic content and 3D? Certain settings like printing workflows might even require the use of large PDFs that contain high-quality images.

So, how best to go about optimizing your PDF documents? First, think about your reasons for producing the document, the identity of your audience, and how they will consume the document. Do you have any special regulatory or accessibility requirements?

The remainder of this article will have a practical look at the nuts and bolts of optimizing PDFs. Rather than try to consider all of the possible usage cases, the following instructional part will focus on reducing PDF file size. I'll outline how to check which elements are contributing most to file size, touch on some of the things that can make PDFs bigger, and then talk about how to trim down your PDFs using Acrobat.

Using the Save As command

As the final step before sending off your PDF for delivery, whatever that means in your setting, use the Save As command (File > Save As). Why? When you perform a normal Save, Acrobat appends any changes to the PDF file. While these changes are invisible in a standard PDF viewer, they still contribute to total file size. Using the Save As command discards this information, rewriting the PDF in the most efficient way possible and reducing its file size. This will also enable Fast Web View, which improves online viewing by allowing viewers to download each page as they read it rather than waiting for the entire document to download.

In short, Save As should always be your last step before sending off your PDFs.

PDF Optimizer

Acrobat has a very handy feature. Well, OK, it has quite a few handy features, but the one I mean now is called the PDF Optimizer. It's pretty seamlessly integrated into the Acrobat interface, so it's possible that you have been using Acrobat for years and either haven't used it or haven't realized that you've already been using it.

Intrigued? Acrobat's PDF Optimizer is actually sort of hidden behind our new friend, the Save As command (File > Save As). If you choose "Adobe PDF Files, Optimized", the greyed-out "Settings" button will become live, and clicking on it opens the PDF Optimizer interface.

If you just choose to Save As an optimized PDF without opening the PDF Optimizer interface, Acrobat will still optimize your PDF based on its current settings, which may well reduce file size. Regardless, it will still efficiently re-write your PDF just like the "vanilla" Save As PDF option. While that's a good start, you may well be able to minimize file size by adjusting the settings some.

Audit space usage

Before you start removing content elements, downsampling images and the like, it's helpful to identify what is actually taking up the most space in your PDF. Luckily, the PDF Optimizer has a feature that can handle just this sort of thing. To activate it, just click on the "Audit space usage..." button in the top-right of the PDF Optimizer window. If you have any trouble finding the button, this video outlines how to find it in Acrobat. You'll then get a dialog that will break down your PDF by the space used by each type of content. It will also tell you the percentage of total file size accounted for by each content type. This will tell you where you can best direct your attention to produce the biggest reductions in total file size.

Using the PDF Optimizer

The PDF Optimizer presents a lot of options. Since Acrobat/PDF Optimizer breaks them down by them into separate panels, we'll do the same here. The types of content to be optimized can be toggled individually using the check boxes next to the panels on the left-hand side of the PDF Optimizer interface. Clicking on the panel name on the left-hand side brings up the panel and its specific options.

Using optimization profiles

You have rather a lot of options when customizing the PDF Optimizer to your particular needs. First up, check the settings for various in-built profiles. By default, Acrobat XI only ships with two: Standard and Mobile. Essentially, Standard is designed for broad compatibility with a broad range of viewing environments. As a result, it is compatible with older viewing software, and isn't too drastic in the way it shrinks images and removes other elements. Mobile, on the other hand, represents a more aggressive approach to optimization, and is designed to ensure smaller file sizes that can be downloaded and viewed on devices that typically operate with more limited bandwidth.

If one of the built-in profiles seems to suit your needs, then great! Use it. If not, then it's easy enough to tweak them. Note that changing any settings will reset the current profile to Custom, but you can then save any custom settings to a new profile, so that isn't really a problem.

Optimizing images (Images Panel)

Images often contribute a significant amount to file size. The key processes involved in optimizing them are compression and downsampling. Compression eliminates redundant or unwanted pixel information, while downsampling reduces the resolution of images to save space. The user can select the compression type, resolution that triggers downsampling, and the resolution to which such images will be downsampled.

PDF Optimizer allows separate settings for color, grayscale and monochrome images. Due to differences in the number of possible colors, these different types of images take different amounts of space, so, for example, a high-res monochrome image would occupy an amount of space equivalent to a much lower-res color image.

The principles of downsampling are relatively straightforward: lower-res images take less space but look less sharp. What might be less obvious is which compression methods are best suited to particular types of images. In general, JPEG and JPEG2000 are best suited for use with images like photographs, where colors tend to change gradually. ZIP can be used with images with more clearly defined palettes and larger areas of solid color, like logos, layout art and some illustrations. JBIG2, CCITT Groups 3 & 4 are best used with monochrome images.

With JPEG, JPEG2000 and JBIG2 compression, the user must also choose a compression quality that offers a suitable trade-off between image quality and file size. Essentially, higher levels of compression discard more pixel information and encode each image as a compact approximation of the original. Lossless compression, which is available with JPEG2000 and JBIG2, retains all pixel information.

Unembedding fonts (Fonts Panel)

To ensure viewing fidelity and consistent editing across systems -- not to mention comply with relevant standards -- it's common to embed entire fonts in PDF files. That said, fonts can take up a lot of space in a PDF, especially if there are a lot of them. Fonts can be safely unembedded if they are installed on the computers of the users reading the PDF documents. In that case, the reader's system just accesses their local copy of the font. Clearly, this is safest when system or other essentially ubiquitous fonts have been used to compose the PDF document.

If the reader doesn't have the font installed, then their PDF viewing software will select a locally-installed substitute font. As such, fonts should still be embedded when a consistent look-and-feel is crucial, unusual or custom fonts are used, or when it is mandated by compliance requirements. File size versus utility is always the trade-off when attempting to optimize PDFs.

Flatten transparency (Transparency Panel)

In PDF files with graphics that contain transparency, the Transparency panel can be used to flatten it. Flattening transparency incorporates it into artwork by sectioning it into vector- and raster-based areas. The Transparency Panel features several presets based on the desired quality. As with images, there is a trade-off between quality and file size.

Editor's Note: The original version of this article incorrectly claimed that flattening transparency could reduce PDF file size. Thanks to two long-time friends of Planet PDF: PDF guru Mike Jahn, for spotting the mistake, and PDF Architect & Principal Scientist for Adobe Systems Leonard Rosenthol, for confirming it.

Remove unwanted elements (Discard Objects & Discard User Data Panels)

The Discard Objects and Discard User Data settings permit the removal of intact but unwanted elements of PDF files. They allow the flattening of form fields or layers, discarding of embedded settings, annotations, interactive elements like bookmarks and the conversion of elements into simpler (and more compact) approximations. For example, interactive forms can be flattened so that form data entered by the user becomes a permanent part of the document. These settings can potentially reduce file size at the expense of functionality.

Since these elements can and often do affect the functionality of your PDF, it's best to be careful when using unfamiliar options. Experimentation is best performed after saving a backup of your original file.

Clean up your PDF (Clean Up Panel)

The Clean Up settings can be configured to manipulate compression on the document level and to remove broken or otherwise redundant elements. This includes things like discarding invalid links and bookmarks, streamlining encoding settings, and optimizing PDFs for fast web viewing. While the latter doesn't specifically address file size, web-optimized PDFs are still quicker to view online, which is often one purpose of reducing file size in the first place.

By default, only options that cannot affect functionality are selected; indeed, there is only one option that is not selected by default, "Discard unreferenced named destinations". Without getting too technical, a named destination is like a beacon in the PDF. Once created, it's possible to point to that beacon either externally (e.g., from another PDF) or internally (i.e., from another part of the same document). This option can only check whether anything within the PDF points to the named destination. In other words, it can only check for internal references. If there are external references to the destination, checking this option will break those links. As with the Discard Objects and Discard User Data settings, it's worth backing-up your original PDF before experimenting with this.

Save the optimized file

Once you have configured everything, save the optimized file. Even if you haven't specifically saved your settings as a profile, Acrobat will still remember your last PDF Optimizer settings. These will be the default configuration the next time you boot up Acrobat.

Optimizing PDFs in batches

It's also possible to optimize archives of PDF documents in large batches using Acrobat's Action Wizard (under Save & Export > Save). Outlining precisely how to do that is outside the scope of this article, but might be the topic of a future how-to. Until then, happy optimizing!

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