OTF stands for OpenType Font. This is usually found on Windows
PCs.
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"Pro" is what Adobe calls the OpenType fonts with extended
character sets.
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Font files can be converted to different formats (TrueType,
OpenType, Web Fonts, etc.) using font conversion software, like
FontXChange for Macintosh and Windows.
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OpenType is a new format over the earlier TrueType and
PostScript Type 1 formats for fonts. As described by Microsoft and
Adobe, it is technically an extension of Microsoft's TrueType Open
format, which contains either PostScript or TrueType font outlines
in a single font file used on both Windows and Mac. It also
includes an expanded character set based on the Unicode encoding
standard as well as advanced typographic intelligences for glyph
positioning and glyph substitution that allows the inclusion of
numerous alternatives glyphs in a single font file.