PDF glossary
ToUnicode map
A table inside a PDF font that maps glyph codes back to Unicode characters, so text can be searched and copied.
Because PDF text is stored as glyph codes, a reader needs a way to know which characters they represent. The ToUnicode CMap provides that mapping for each font.
When it's missing or wrong, copying text from the PDF produces gibberish even though the page looks fine.
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