This report is also available as an Acrobat file.
Authoring and Design for the WWW
Appendix Three: Adobe Acrobat
Acrobat was designed to serve two purposes:
- To allow paper documents to be presented on screen,
particularly for example in proofing a layout which might need to be
electronically transmitted to the other side of the world. This it can do without
the end-user having either the correct fonts or the application which generated
the original pages (for example Quark Xpress, PageMaker, FrameMaker).
- To enable the production of on-screen documents. This is
Acrobat puts emphasis on the appearance of the page and
in doing so takes the fixed formatting approach. It contrasts strongly with
HTML which emphasises the structural components of the content, and which is
currently not capable of specifying exactly how the page should look.
We can compare Acrobat's approach to the four problems
in interactive electronic publishing which we identified in discussing HTML.
Proprietary hardware and software formats
Adobe's PDF file format is open and published, but not an
agreed standard. Its design represents Adobe's view of what should be possible.
However, we should recall that Postscript, without which high-quality desktop
publishing might not have been achieved, was (and still is) an Adobe
proprietary format. PDF files must be read with the Acrobat reader, which can
be distributed free by the document publisher.
Fonts are property
Acrobat PDF documents can contain embedded definitions
of the fonts needed. Property is protected in that the end-user cannot extract the
fonts and use them in another document (Adobe are themselves major font
publishers). Alternatively, when a document which does not contain embedded
font information arrives at the user's machine, an approximation of the font is
created on demand, using Adobe Multiple Master fonts.
Users constrained by hardware
Unlike HTML, Adobe documents have pages with actual
dimensions, for example A4, and are in that sense virtual paper'. Rather than
the document being fitted to the available space on the display, the user scrolls
the document in the Acrobat viewer window (or sub-window within a Web
browser). This scrolling can be avoided, at least for standard monitors, by
designing pages which are the correct format for the screen but this approach,
however desirable, clashes with the temptation to simply transfer wholesale to
the screen existing documents designed for paper.
Documents self-contained, unconnected
Initially, PDF files could only link in hypertext fashion to
other documents stored on the same server, and were therefore useless as
components of distributed hypertext systems. However, the links embedded in
PDF files can now be true URLs, allowing proper addressing of distant
The PDF format is still extremely limited in its ability to
create proper, maintainable hypertext links.
Features of the package
At its simplest, the user can 'print' a file to the PDF Writer
(a virtual printer), which then makes a Portable Document Format file, which
can be viewed on any platform for which an Acrobat Reader is available
(Windows, Macintosh, DOS, Unix). The reader for Macintosh and Windows
can be distributed free of charge. When the end-users open a document with the
Reader, they can read it page by page, jump to any page, search for any word or
phrase, and view the pages at a variety of degrees of magnification.
If the publisher wishes to provide a greater range of
functions to the user, then they must use Acrobat Exchange. This application
allows the authors to open a PDF file made with the PDF Writer and augment it
in certain ways.
- creating a table of contents ( bookmarks') from
which the user can pick by clicking the pointer
- adding annotations in the form of electronic post-it
notes, which may be put anywhere on the page'
- creating links from any part of any page to another
location within the same document or to a location in another document
- creating 'articles'. These enable end-users to follow a
particular sequence of information in the document without needing to find
for themselves the location on the page of the next block. This is useful for
multi-column documents, where the next part of the story' is at the top of the
next column, rather than the beginning of the next page.
Other facilities in Acrobat Exchange include document
security and facilities for deleting and adding pages, substituting pages, merging
separate documents etc.
Using the package Acrobat Catalog authors can also create
an index to enable faster searching by the end-user.
Some weaknesses of the Acrobat approach
The strengths of Acrobat are plain: the ease of production
and the ability to guarantee what the end-user will see. Some of the weaknesses
listed here may be avoided by future versions.
Poor facilities in the tool-set
None of the facilities for authors are automated to any
useful extent within the Exchange package. Bookmarks can only be created
manually one-by-one. Links must each be built by hand (whereas in HTML
even the simplest word-processor could be used, say, to turn every occurrence
of the word University' into a hypertext link to another document).
Other packages can produce PDF files directly, without
resorting to the PDF writer, and in these cases, automation of certain functions is
greatly improved. For example in Adobe PageMaker (Version 6) if a table of
contents has been made for the publication, then, when a PDF file is made, this
table can optionally be converted into the PDF bookmark list.
Document editing and maintenance
Hypertext links in PDF files are specified graphically, that
is by their position on the page, not by the element such as a word or a
picture which is to act as the trigger. At first site this seems unproblematic, since
a strength of PDF is its ability to specify robustly exactly where items appear on
each page. However, if we now imagine the creation and editing of a multipage
document, the problems start to become apparent. If we create link hotspots
(which we have to do by hand) on several pages, we have created a very fragile
system, since if we edit the original and re- print' the PDF file, text flow has
caused the text to shift away from under its hotspot, and this will be true for
very hotspot on all the succeeding pages. This is exactly the danger we
highlighted in discussing the weakness of fixed formatting approaches (p29).
Technically, this problem is not insoluble for future
versions. However, using the current way in which Acrobat works, it effectively
prevents the production of robust, maintainable documents.
In some respects, Acrobat's ease of transition from paper
to screen-based documents can be seen as a problem rather than a virtue. It
means that many documents will be produced which have been neither written
nor designed for the screen. Parallel efforts by other manufacturers, for example
the facility of the Microsoft Internet Explorer to directly display Microsoft
Word and Excel documents without modification, should also be treated with
care. It is important to distinguish between documents designed for on-screen
interactive use and those pieces of virtual paper which are appropriately
presented on screen as a representation of paper documents. Each medium has
its strengths, and the Web will evolve its own genres with their own styles and
modes of address, beyond the display of would-be paper.
Virtual Environments Visualisation