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What is Acrobat?

Acrobat is the name for a family of document interchange products written by Adobe Systems, Inc. The underlying file format is the Portable Document Format (PDF). The idea is that any document you would normally print, you can now instead turn into PDF, which represents the exact appearance of the printed document. The PDF file can then be viewed by anyone with an Acrobat Reader. Text can be cut out of a PDF file in Rich Text Format (RTF) but the document cannot be edited in any real sense.

Since PDF is platform-independent, and reading and writing software is available for a variety of platforms (Windows, Macintosh, various flavours of UNIX), documents can be exchanged freely between users of those platforms.

As well as representing the printed pages of your document, Acrobat supports additional navigational aids such as hyperlinks, bookmarks and thumbnail views. Acrobat has now reached version 2.1. Version 2.1 readers can read version 1 documents fine, but version 1 readers may miss out some of the extra features in version 2.1 files.

Making Acrobat Documents

To make Acrobat (PDF) documents you need either PDFWriter or Distiller. PDFWriter is like another printer driver, but it writes out PDF files instead of printer commands. Distiller converts PostScript files to PDF. Both cost money, though PDFwriter is quite cheap. Distiller is more expensive, but can handle documents with embedded EPS files and even be used to support automatic hyperlink creation.

Reading Acrobat Documents

Acrobat Reader is now free. It allows PDF files to be viewed and all the standard extra features (links etc.) to be used if present in the file. Acrobat Exchange is like the Reader, but also allows links, bookmarks, annotations etc. to be added to PDF files. With Exchange, pages can be re-ordered, deleted and imported from other PDF files, and plug-in extensions can be added. Exchange costs money.


What is PDF?

PDF is the file format on which Acrobat products rely. It is based on Level 2 PostScript, but as Acrobat Distiller's name suggests, it uses a simplified and limited set of operators. No new operators can be defined and there are no iterative constructs such as for, loop and repeat.

A file is structured as a number of separate objects which may refer to each other. For example, a page object refers to various resource objects (fonts etc.), and links associated with the page, as well as the actual stream of operators which draw the page.

These objects are numbered and may appear anywhere in the file. To enable quick random access, a cross-reference table at the end of the file says where each object is, and which object froms the root of the file.

The first version was 1.0. Version 2 Acrobat products use the additional functionality of PDF-1.1.


Font Handling

Acrobat is designed so that users rarely have to worry about fonts. This of course means that this side of Acrobat software has to be quite clever, but the basic idea is as follows: `Standard' fonts (Adobe Times, Helvetica, Courier, Symbol and Zapf Dingbats) are available with every Acrobat installation. For other fonts, when the PDF file is produced, a description of character widths, weight and style is included. When the file is viewed (possibly on another platform), if the original font is not available, a substitute is made up from the information in the file. In most cases this is good enough; lines are the right length, characters don't crash into each other and the overall appearance is similar to the original. However, if it essential to the author that every reader should see the correct font, the whole font (or a subset) may be embedded in the PDF file at generation time.

PDF Corner