Web Site Optimization: How to Optimize PDF Files for Web Sites
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Portable Document Format (PDF) is the defacto file format for presenting device-independent documents on and off the Web. While PDFs have become quite popular on the Web, many PDFs used in web sites are designed for high quality print output and are not optimized for the Web.Even PDFs designed for Web use can have a wait problem, weighed down with excess fonts, change histories, and unoptimized images and forms. Optimizing PDF files for the Web can significantly shrink their size and boost display speed, saving bandwidth and user frustration. (For the full “Optimize PDF Files for the Web” article, see http://www.websiteoptimization.com/speed/tweak/pdf/
Creating Small PDFs
The main factors in creating small PDFs are image resolution, image type (bitmap or vector), the number of fonts used and how they are embedded, PDF version, and the level of compression. In general the higher the PDF version number, the smaller the file. Acrobat 5 (PDF version 1.4) added JBIG2 compression, which is superior to the CCITT or Zip algorithms when compressing scanned monochromatic copy.
JBIG2 (Joint Bilevel Image Experts Group) encoding compresses monochrome (1 bit per pixel) image data from 20:1 to 50:1 for pages full of text. Like other dictionary-based algorithms (LZW, ZIP) JBIG2 creates a table of unique symbols and when a subsequent symbol matches one in the table, it substitutes a token pointing to the table index. JBIG2 also compresses the entire table.
Acrobat 6 (PDF version 1.5) added the ability to compress the entire file (Clean Up Settings dialog). However, since over 90% of Acrobat users have version 5.0 or greater, using PDF 1.4 is a safer alternative. Acrobat will usually display (with a warning) a more recent PDF version, but new compression schemes will spawn an error when opened in older versions of Acrobat. At the time of this writing, Adobe says that of those 90%, 50% use version 5 and 40% use version 6.
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