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PDF: The Cure for Dawdling Digital Files

(December 1999) posted on Tue Dec 14, 1999

Learn the basics of creating PDF files.

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By Mark A. Coudray

For embedded fonts to work with PDF files, the screen and printer fonts used to create the graphic must both be available to Acrobat when the file is printed to PostScript. When the type is embedded, all font outlines, metrics, and sizes are also embedded in the document. This allows the document to display correctly on the computer screen and print without including the fonts with the file. And when a font is embedded, all of the outline information for all characters in the embedded font family are included in the document, regardless of whether they are used or not.


Subsetting fonts is a less desirable option, but it results in a smaller file size because only the characters used in the document are included in the subset. This approach can be somewhat limiting if we need to edit the distilled PDF file and the new character we need has not been included in the file. Additionally, Acrobat substitutes special serif and san serif faces if font information is missing, but preserves the desired character spacing. Font substitution also takes place if bitmapped fonts are used in the original file before it's distilled.


Transmission and storage of PDF files


The PDF workflow involves converting all continuous-tone images, vector graphics, bitmap graphics, and fonts to PostScript code, then using Distiller to compress and write the code into PDF format. The resulting PDF files are inherently smaller than conventional files. But how much smaller depends on the compression option we select when running Distiller.


One of the important distinctions with PDF is that the compression of file information is variable; we can apply different levels of compression to different types of data within the same document. This results in the smallest possible document size, with little or no loss of image quality. The menu of compression methods Acrobat provides includes JPEG, LZW, ZIP, and Run Length Encoding.


JPEG Acrobat's JPEG option offers five user-selectable levels of compression. Since JPEG is a lossy compression method, the greater the compression selected, the greater the loss in image quality. JPEG offers some of the highest compression ratios and, consequently, some of the smallest file sizes. It works best on continuous-tone data.



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