OpenType

An OTF file is an OpenType font, the format behind most typefaces sold by type foundries and installed on current computers. You typically get one in a ZIP from a foundry, from a free font library, or in the fonts folder of a design project someone handed over. Windows, macOS and Linux install OpenType fonts without any extra software.

Where OpenType came from

In the early 1990s fonts were split into two camps: Adobe's PostScript Type 1 (PFB, PFA) and TrueType from Apple and Microsoft. Microsoft and Adobe announced OpenType in 1996 as a single format that can carry either kind of outline. Microsoft publishes the OpenType specification (version 1.9.1 at the time of writing), and the same technology is an ISO standard, ISO/IEC 14496-22 "Open Font Format". Version 1.8 from 2016 added variable fonts: one file that holds a whole range of weights or widths instead of a separate file per style.

What is inside an OTF file

An OpenType font is a set of tables in one binary file. The table that matters most for conversion is the one holding the outlines:

• CFF outlines (cubic Bézier curves, the PostScript way). Files with these outlines start with the bytes OTTO and conventionally use the .otf extension.
• TrueType outlines (quadratic curves). These usually end in .ttf, although they are OpenType fonts as well.

Other tables map up to 65,535 glyphs to Unicode, store kerning and the layout features that switch on ligatures, small caps, old-style figures or the contextual forms that Arabic and Indic scripts need. The OS/2 table carries the embedding permissions set by the vendor: whether the font may be embedded in a PDF or document for editing, only for viewing and printing, or not at all.

How to open and install an OTF font

• Windows 10 and 11: right-click the file and choose Install (or Install for all users). Restart Word or Photoshop afterwards, many programs read the font list only on start.
• macOS: double-click to preview it in Font Book, then click Install.
• Linux: copy the file to ~/.local/share/fonts and run fc-cache -f, or use the font manager of your desktop.
• Editing: the free FontForge runs on all three systems. Professional type designers mostly use commercial editors.

Common problems with OTF files

Windows says the OTF is not a valid font file

The download is often a web font with a wrong extension or an HTML error page saved as .otf. Open the file in a text editor: a real OTF begins with OTTO or with binary zeros, a WOFF file begins with wOFF. A WOFF has to be converted before Windows will install it.

The font does not show up in Word or Office

Close and reopen the program. For variable fonts, Office and some older applications list only the named styles defined in the font, not every weight on the axis.

Text set in the font cannot be embedded in a PDF

That is the embedding permission in the font, chosen by the vendor. It is part of the licence terms and converting the file does not change it.

Converting OTF to other font formats

Usual reasons: TTF for software that accepts only TrueType (some older programs, cutting plotters, embroidery software), WOFF or WOFF2 for @font-face on a website, Type 1 PFB or PFA for an old RIP or a legacy TeX setup. Our converter rebuilds the font from its outlines. What to expect:

• OTF to TTF: cubic curves are approximated by quadratic ones. The difference is invisible in print, but the original PostScript hints do not carry over, so small text on screen can render slightly differently.
• OTF to PFB or PFA: Type 1 addresses at most 256 characters through its encoding and has no OpenType layout features. Ligatures and contextual forms are lost, and kerning goes to a separate AFM metrics file, which we also offer as a target.
• Variable fonts: the result is a static font with a single style.

A note on licensing. Converting a font does not change its licence. Many commercial desktop licences forbid modifying the font or using it on a website, and web use often needs a separate web licence. Fonts under the SIL Open Font License may generally be converted and embedded. Check the licence before you publish a converted font.

OTF at a glance