PDF glossary
Font subsetting
Embedding only the glyphs a document actually uses instead of the whole font, to keep the file small.
A complete font can contain thousands of glyphs. Most PDFs embed only those used in the document. You can recognise a subset by its name: six capital letters and a plus sign, like ABCDEF+Calibri.
Subsetting saves space but means the embedded font can only draw the characters that were already in the document.
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