PDF In-Depth

Best Practices #2: Creating Effective Acrobat Bookmarks

August 27, 2001

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Editor's Note: This is the second in a series of short, focused articles designed to help users create optimal PDF files -- and to understand the various factors involved in doing so.

Acrobat's bookmarks can be a particularly efficient navigation mechanism that provides easy access to common locations and actions. With nesting and expand/collapse capability, bookmarks create a constant conceptual display of the document's structure. Starting with Acrobat/Reader 5, bookmarks can have color/font style properties, so that specific bookmarks can be emphasized.

Bookmarks do not complicate display and printing aspects of the PDF file (as they, together with all interactive features, are ignored when the file is printed). Bookmarks are either created automatically when using common authoring applications (PageMaker, FrameMaker, or Word with PDFMaker, to name a few) and/or manually in Acrobat. However, in both cases, efficient bookmarks are not guaranteed. This article provides guidelines for bookmarks that are useful to the readers of PDFs.

What Should Be Bookmarked?

Bookmarks should be available for all important items in your PDF. In business or technical documents, bookmarks typically include up to 4 top-level headings. As a minimum, all top items should be included: chapter headings, all frequently accessed locations such as Contents, Index, List of Figures or Tables (which may be used to locate the required item and jump directly to it using a link).

When producing a document collection, be consistent across the collection and also add bookmarks leading from one document to the others (such as main menu and/or other chapters).

While bookmarks commonly point to destinations within the current PDF, they can also point to other PDFs, and can be used for other related purposes:

  • Linking to web sites and e-mail addresses

  • Launching external data files

  • Specifying various Acrobat actions, for example: Search (all Acrobat actions can be assigned to bookmarks)

  • Linking to multimedia items (video/sound)

Does a list of bookmarks replace a table of contents?

No. Bookmarks are inefficient when expanded to show the entire hierarchy, since they become a very long list that must be scrolled. In addition, expanding the bookmarks (if initially collapsed) has to be done manually for each and every bookmark.

A detailed table of contents provides a listing of all topics at one glance, whether it is one page long or spans several pages. Many readers inspect the table of contents when opening a book - printed publication or an online document - trying to find items or understand the structure. The table of contents should therefore provide a listing of all topics. The bookmarks can be a subset of the table of contents with fewer levels, or identical in depth. However, it makes no sense for the table of contents to be less detailed than the bookmark list. Supplying both bookmarks and a table of contents provides the reader with multiple options.

Another consideration is that many PDFs are printed, in which case a Table of Contents is essential. Often, with huge documents, even if the PDF itself is not printed, the Contents pages may be printed and kept for handy reference.

A bookmark to the table of contents is essential. Even if the table of contents is at the beginning of the document, a cover page, disclaimer, warranty and other front matter items typically precede it. A Contents bookmark alerts the reader to the existence of the table of contents and makes it easy to access regardless of the page currently open.

The table of contents must also have hyperlinks, so that the user can jump directly to the topic chosen, without having to locate the topic in the bookmarks.

Order of Bookmarks

Bookmarks to headings should typically reflect the order of the items in the book. However, there are exceptions to this rule. When there are too many bookmarks and the entire list cannot be displayed without scrolling, even when all top items are collapsed, a bookmark to the index may be hidden at the end of the list. In such cases, it is recommended to place the Index bookmark at the top of the list, perhaps after the Contents bookmark. (It is also helpful to have subordinate bookmarks to letters within a lengthy index, so that it is easy to jump to entries starting with "M", for example).

In publications with many titled tables and figures, specific bookmarks for each of these may be useful. However, when all items are in the order of appearance in the document, bookmarks to figures or tables will be spread, showing under headings of different levels, most of which are likely to be collapsed. Creating separate "Figures", "Tables" or "Equations" bookmarks with all relevant bookmarks subordinate to them is much more efficient, similar to such lists in printed books.

Easy to Identify

Bookmark text should be easy to identify. The bookmark pane is relatively small and in most cases will not be wide enough to display complete headings. Starting with Acrobat 4, a tool tip for the bookmark being pointed to is displayed, showing the full text if it is too long to fit in the bookmark pane. But despite this useful feature, the more meaningful text can be displayed, the better.

Common situations that cause the actual content of the bookmark text to be partially pushed out of view include:

  • Insignificant or non-unique text at the beginning, such as The, Chapter, Appendix (which could be shortened or omitted, for example "1: Filing Principles…" instead of "Chapter 1: Filing Principles …", or "B: System Specifications…" instead of "Appendix B: System Specifications").

  • Heading numbers - despite the usefulness of heading numbers in the document itself, both for locating material quickly and for easy comprehension of the hierarchy level, in bookmarks the text is much more important. Numbering, particularly in lower-level bookmarks of documents that use multi-level numbering (which have less space due to indentation), may use a significant part of the bookmark pane. And, if you have included a table of contents in your document, the numbering will be present there and can be omitted in the bookmarks.

  • All-caps text causes text to be less readable. While this is true with regard to headings too, some style guides require all caps for top-level items. Depending on the authoring software being used, it may be possible to have the text in the page shown as all caps, while the bookmarks can use upper- and lower-case text.

Open Mode, Hierarchy and Initial Collapse/Expand State

Generally, if bookmarks are created for a PDF, it is a good practice to have an initial view of "Bookmarks and Page" - otherwise some readers may not be aware of the availability of the bookmarks.

The bookmark hierarchy should match the document's heading hierarchy, unless you have a very good reason for breaking this rule.

It is also important that the opening collapse/expand state displays the main items for the entire publication, so that top-level items are visible, with other levels collapsed - resulting in a much neater structure. In addition, when all or most bookmarks are expanded, the list is too long and most of it is hidden initially, so that important items may never be used (as scrolling long lists is not typically done). In bookmark structures where "Chapter" bookmarks are the top ones, bookmarks should be initially collapsed under the first level; in structures where there are "Part" or "Book title" bookmarks as the first level (with chapters as the second level) - bookmarks should be collapsed under the second level.

When saving a PDF in Acrobat, the current state of bookmarks is preserved. Thus, it is important to pay close attention to the collapse/expand state just before the "last" Save (or better yet, Save As). Of course, exceptions to the collapse/expand level set automatically by the authoring application can be made, as long as these are intentional!

Bookmarks use a single system font; it is not possible to have variations in fonts/styles within a bookmark. Some special characters may suffer as a result - Greek letters or scientific symbols in a heading, for example, will be carried over to the bookmark text as characters in the standard font. While Acrobat itself supports Unicode encoding in bookmarks since version 4.05, authoring applications are slow to implement the required mechanisms to support this.

Control characters should not be included in the bookmark text, yet some authoring applications (particularly FrameMaker) transfer whatever is included in headings when bookmarks are created automatically. Common control characters include non-breaking spaces, special fixed spaces, non-breaking hyphens, suppress hyphenation characters and new line characters. Version 3 and 4 of Acrobat either ignored such characters in bookmarks or displayed them as spaces. In Acrobat/Reader 5, they are displayed as boxes.

This is true even when a PDF was created using Distiller 3 or 4 and is being displayed in Acrobat Reader 5. Beware of this unsightly phenomenon that may appear in some of your old PDFs when users open them with Reader 5.

Color and Font Properties

One of the few additions to Acrobat 5.0 as far as users of Acrobat Reader are concerned is bookmark color and font style properties. Unique colors/styles are useful as a way to highlight special bookmarks (such as What's New), or to designate bookmarks with external destinations (other files or web links). Using these properties can also be helpful as a way to distinguish between hierarchy levels.

Spelling

This may sound obvious, but there is nothing more embarrassing than a bookmark with a typo, displayed at all times. When bookmarks are created automatically from paragraphs, we can assume that the document itself was checked for errors before output to PDF. When bookmarks are added manually, double-check the bookmark text. (When creating a bookmark to a heading manually in Acrobat, the paragraph can be selected first so that its text is retrieved without having to retype it).

Remedies

Many of the above recommendations do not necessarily have to be implemented manually in Acrobat. Maximize the use of settings provided with your authoring tool and use add-ons that automatically remove numbering and redundant text, add color/style to your bookmarks, control initial collapse state, handle special characters or reorder figure, table or equation bookmarks.

Review of the use of Bookmarks in PDFs in the Acrobat 5 CD

The Acrobat 5 CD contains various PDFs, including the Acrobat help files and the Acrobat SDK documentation (Acrobat SDK is also available in partners.adobe.com) - 42 PDF files in total.

With one exception, all PDFs were authored in FrameMaker, which is capable of generating bookmarks from designated paragraph styles (similar or better capabilities are available with other popular authoring tools). With a few exceptions, it does not seem that bookmarks were given thought beyond their automatic rendering. Automation is great, but care has to be taken to make the most out of the settings available; if necessary, additional techniques - manual or automatic - have to be employed.

Inspecting the use of bookmarks in the Acrobat 5 CD PDF collection, I reached the conclusion that it was not inspiring. Review the same PDFs and make up your own mind!

Bookmark composition and style is not consistent in the different PDFs, even though many are very similar in structure:

In about two thirds of the PDFs, the document's title is used as the first bookmark.

  • Bookmarks to Contents are available in most PDFs but not in all (e.g. Spelling API, ADM Reference Guide). The pdfmark reference manual is present twice on the CD - once as part of the SDK and once in Acrobat Help documents; one version has a Contents bookmark, the other does not.

  • The Acrobat SDK has an interactive Roadmap chart included in most of documents. In most PDFs where the Roadmap is present, there is a bookmark pointing to it, but a few PDFs (Acrobat Development Overview, Core API Overview, Distiller API Reference) do not have one. (This was fixed in SDK PDFs available for individual download, dated June 25)

  • The 2-page index in the PDFMaker help PDF file has a bookmark with subordinate bookmarks for the index letters, whereas the bookmark pointing to the 17-page index in the Acrobat Core API Reference does not have these.

  • The treatment of the top-level numbered bookmarks is different in otherwise-identical PDFs included twice in the CD (Acrobat Distiller Parameters, present once in the SDK and once in the Help).

Bookmark nesting and collapse/expand state also vary. This may be related in part to the way FrameMaker generates bookmarks, with a default collapse/expand state that seems whimsical (there is some arbitrary logic behind it, having to do with the number of subordinate bookmarks). Nonetheless a simple fix in Acrobat could have remedied this.

In a few PDFs, collapse/expand state seems to be "random" (that is, the bookmark state used by the author when the PDF file was last saved).

One could expect that PDFs supplied with Acrobat 5 would be taking advantage of the new bookmark color/style properties. I was able to find only one bookmark that looked different: the "What's New For 5.0" bookmark in the Acrobat JavaScript Object Specification PDF.

All bookmarks in the Acrobat 5 CD PDFs point to paragraphs in the current PDF. Acrobat functionality does not limit bookmarks to this purpose, and the document creators could have added:

  • Bookmarks to the Acrobat News or Technical Updates web pages, where readers can possibly learn about a new version of the document available for download

  • Bookmarks that when clicked open a feedback form that can be e-mailed instantly using the user's mail client

  • Bookmarks to other PDFs (for example, the Roadmap could have been produced professionally once and then linked to by bookmarks in all other PDFs, instead of being physically duplicated in each manual, which has a undesirable side-effect when using Search)

  • A "Popular Searches" bookmark, with subordinate bookmarks that have commonly-used pre-defined Search terms. Click the bookmark, and you get an instant Search Results for the pre-defined item.

Specific Examples

Acrobat Help - AcroHelp.pdf

When opening the Acrobat Help file, four top-level bookmarks are visible: Using Help, Contents, Index and Legal Notices. To see specific topics, one has to click the Contents bookmark, and repeat this each time the PDF is opened. It would have made sense to save the PDF with the Contents bookmark expanded, so that second-level items under it are visible with no need for an additional click, or to otherwise set the two top levels bookmarks to be visible.

The Contents bookmark in the Acrobat Help file, when clicked, takes us to a single page Contents, which only lists the chapters - so that a comprehensive list of all topics in the document is not available, unless one is willing to expand all bookmarks.

An example for a box in the bookmark text is available under Contents > Creating Adobe PDF files > Converting Asian Text to Adobe PDF > Working with Japanese ATM Type 1 Fonts.

PDFMaker help PDF - Help/ENU/PDFMaker.pdf

In the PDFMaker help PDF, which has a structure similar to the Acrobat help PDF, the chapter bookmark is subordinate to the collapsed Contents bookmark. Not much logic here…

PDF Reference Manual, 2nd Edition, v.1.3

When opening the PDF Reference manual included in the Acrobat 5 CD, the bookmark branch starting with Appendix H is entirely expanded, for no good reason.

Incidentally, in that same expanded Appendix H, under "H.3 Implementation Notes," several bookmarks show boxes for various reasons, including non-breaking spaces (1.2, Type 4), discretionary hyphens (4.8.4, Dictionaries), suppress hyphenation characters (4.9, X objects).

The three-level numbering carried to the bookmark text is not a wise use of the prime real estate of the bookmark pane.

Core API Reference - Core_API/CoreAPIOverview [PDF: 17 MB]

The Contents bookmark in the 2755-page long Core API Reference points to a confused table of contents slightly longer than one page, which lists only the main categories (with items of different levels using the same format). To make the barely useful Contents page even less useful, the few items that it has are not hyper-linked (links will be treated in a separate Best Practices article).

Summary

There are many different ways in which the bookmarks in the Acrobat 5 PDFs could have been more useful, attractive and easier to use. The limitations demonstrated in these PDFs are not related to Acrobat's capabilities as a tool, but to the lack of someone taking a bird's eye view of the collection as a whole and making decisions to be followed consistently.

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