PDF glossary
.notdef glyph (tofu)
The fallback shape a font draws when it has no glyph for a code, usually an empty box nicknamed tofu.
Every font has a .notdef glyph at position zero. When text asks for a glyph the font doesn't contain, readers draw .notdef, often a rectangle or nothing at all.
Tofu boxes in an edited PDF are a sign that the editor wrote characters the embedded subset can't draw.
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