Previous Next

                                               39
     SECTION 2.2                                                   Other General Properties



     Any PDF file can also be represented in a form that uses only 7-bit ASCII
     (American Standard Code for Information Interchange) character codes. This is
     useful for the purpose of exposition, as in this book. However, this representation
     is not recommended for actual use, since it is less efficient than the normal binary
     representation. Regardless of which representation is used, PDF files must be
     transported and stored as binary files, not as text files. Inadvertent changes, such
     as conversion between text end-of-line conventions, will damage the file and may
     render it unusable.


2.2.2 Compression

     To reduce file size, PDF supports a number of industry-standard compression
     filters:

     • JPEG and (in PDF 1.5) JPEG2000 compression of color and grayscale images
     • CCITT (Group 3 or Group 4), run-length, and (in PDF 1.4) JBIG2 compression
       of monochrome images
     • LZW (Lempel-Ziv-Welch) and (beginning with PDF 1.2) Flate compression of
       text, graphics, and images

     Using JPEG compression, color and grayscale images can be compressed by a fac-
     tor of 10 or more. Effective compression of monochrome images depends on the
     compression filter used and the properties of the image, but reductions of 2:1 to
     8:1 are common (or 20:1 to 50:1 for JBIG2 compression of an image of a page full
     of text). LZW or Flate compression of the content streams describing all other
     text and graphics in the document results in compression ratios of approximately
     2:1. All of these compression filters produce binary data, which can be further
     converted to ASCII base-85 encoding if a 7-bit ASCII representation is required.


2.2.3 Font Management

     Managing fonts is a fundamental challenge in document interchange. Generally,
     the receiver of a document must have the same fonts that were originally used to
     create it. If a different font is substituted, its character set, glyph shapes, and met-
     rics may differ from those in the original font. This substitution can produce un-
     expected and unwanted results, such as lines of text extending into margins or
     overlapping with graphics.

Previous Next