4.5.3 Page oriented solutions

Postscript

Postscript was invented by John Warnock and Charles Geschke, co-founders of Adobe, in the early 1980's. It is a formal programming language that is specialised for producing print output. Postscript is not normally something that end-users interact with directly. The software that they use generates a Postscript file when they print their document. This Postscript document is then sent over a network connection (or rarely a direct connection) to a printer (usually a laser printer, although some high-end inkjet printers also have Postscript as an option). The printer contains an implementation of Postscript in Read Only Memory (ROM) and a Central Processing Unit (CPU) of its own. The printer takes the Postscript code and runs it as a program. The output of this program is dots placed precisely on pages that are then output.

Postscript, in its first implementation inside Apple's Laserwriter printers, was largely responsible for starting the desktop publishing revolution. For the first time, designers could product sufficiently complex output on (relatively) affordable printing hardware.

Postscript is still used as a medium for distributing documents on the Internet, particularly within the physics, engineering and computer science communities. Naturally, the user needs a Postscript printer or a Postscript interpreter written in software (such as Ghostscript) to print or view these files.

Adobe's Portable Document Format (PDF)

Postscript suffers from a number of problems as a universal electronic document format:

• the files tend to be fairly large (perhaps 50 K per page)
• fonts used are restricted to the 13 fonts that are known to be built into all Postscript laser printers
• files are not easily viewable on screen
• files are not easily searchable
• the readers require a Postscript printer (or software interpreter) for output
• the electronic documents are basically print-only; they cannot have any additional navigation/viewing features built into them.

The Portable Document Format (PDF) was invented by Adobe as a solution to these difficulties. PDF provides:

• resolution-independent output
• fixed page-size documents
• a range of navigation mechanisms
• searchable text
• the exact electronic equivalent of the equivalent print document
• links to Web sites
• security restrictions on use (no copy/no print) built into the PDF file
• a range of compression techniques
• ability to include fonts or font subsets into files
• much smaller file sizes than Postscript

PDF (sometimes misleadingly called Acrobat) can be authored in three ways:

• Use a PDFWriter printer driver that intercepts the print stream and produces PDF output
• Use the Acrobat Distiller program that converts Postscript output from any program into PDF
• Use an Adobe program (Pagemaker, Framemaker, Photoshop, etc.) that can produce PDF directly

Once the PDF has been created the Acrobat Exchange program allows direct editing of PDF, importing of images, and the creation of navigational overlays. It is also possible to annotate Adobe PDF files but only with the full Acrobat Exchange product. This means that very few readers of PDF can make use of this facility. It is important to note that PDF is a proprietary product (although its specification is openly available) and the authoring programs are not free.

In order to access PDF files, the user needs the Acrobat Reader software which is freely available for MacOS, Windows, some Unix flavours. This reads PDF files and renders them on screen or sends them to any printer for output. If the creator of a PDF file used a font not on the reader's computer and did not embed that font in the PDF, the Reader can use Adobe's Multiple Master technology to create virtual fonts that preserve the original line breaks and spacing.

The easiest way to think about PDF is as electronic print . It allows content creators to generate documents which match as far as possible the print originals. In fact, when printed on a laser printer they are usually indistinguishable from photocopies of the print originals. Because PDF is based on Postscript (it is essentially a specialised dialect of Postscript) it fits easily into the print production processes of many publishers.

John Warnock likes to talk about PDF as representing the shift from 'Print then Distribute' to 'Distribute then Print'. In other words, creators of content no longer have to have warehouses full of printed materials that they have to physically move to their users (and which become out of date, cost money to store, can't be easily changed, etc.). Creators simply have to make available a faithful electronic version of their document and let the users print it themselves (if they choose to). This, of course, presupposes two things: that people have ready access to a printer, and that they are prepared to accept the lower resolution associated with desktop printers (300 or 600 dpi as opposed to 1250 or 2500 dpi for professional publishers). The first of these is a fairly safe assumption in most corporate environments (and increasingly in home environments as well) in the developed world, and the second depends on the type of material being produced. Colour printers are still fairly rare, but much publishing only requires monochrome output and 300 dpi is usually adequate if not optimal. (see 4.2.3: Print output on page 53).

PDF is an excellent choice for documents with complex formatting requirements and in situations where exact page fidelity is required. Because of the font embedding and freely available reader it largely avoids many problems with cross-platform documents. For all these reasons, PDF is rapidly becoming a standard option for publishers who are producing parallel print and electronic versions of their publications. A typical scenario is for such publishers to make Web versions of their articles available for viewing on screen with a PDF version to download and print out. The printed PDF will usually display the identical formatting (down to page numbers) of the print original and will probably be of a higher quality than an inter-library loan photocopy.