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    SECTION 5.7                                                          Font Descriptors



    CID. If a matching notdef mapping is found, the CID selects a glyph in the associ-
    ated descendant, which must be a CIDFont. If no glyph exists for that CID, the
    glyph for CID 0 (which is required to be present) is substituted.

    If the CMap does not contain either a character mapping or a notdef mapping for
    the code, descendant 0 is selected and the glyph for CID 0 is substituted from the
    associated CIDFont.

    If the code is invalid—that is, the bytes extracted from the string to be shown do
    not match any codespace range in the CMap—a substitute glyph is chosen as just
    described. The character mapping algorithm is reset to its original position in the
    string, and a modified mapping algorithm chooses the best partially matching
    codespace range:

    1. If the first byte extracted from the string to be shown does not match the first
       byte of any codespace range, the range having the shortest codes is chosen.
    2. Otherwise (that is, if there is a partial match), for each additional byte extract-
       ed, the code accumulated so far is matched against the beginnings of all longer
       codespace ranges until the longest such partial match has been found. If multi-
       ple codespace ranges have partial matches of the same length, the one having
       the shortest codes is chosen.

    The length of the codes in the chosen codespace range determines the total num-
    ber of bytes to consume from the string for the current mapping operation.


5.7 Font Descriptors

    A font descriptor specifies metrics and other attributes of a simple font or a
    CIDFont as a whole, as distinct from the metrics of individual glyphs. These font
    metrics provide information that enables a consumer application to synthesize a
    substitute font or select a similar font when the font program is unavailable. The
    font descriptor may also be used to embed the font program in the PDF file.

    Font descriptors are not used with Type 0 fonts. Beginning with PDF 1.5, font de-
    scriptors may be used with Type 3 fonts in Tagged PDF documents (see Section
    10.7, “Tagged PDF”).

    A font descriptor is a dictionary whose entries specify various font attributes. The
    entries common to all font descriptors—for both simple fonts and CIDFonts—are

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