PDF glossary
Font subsetting
Embedding only the characters a PDF actually uses from a font, instead of the whole font — smaller files, but no glyphs for new text.
Most PDFs embed subsets: if a word never appears, its letters may be missing from the embedded font. That is why changing the text of a PDF can need letters the document’s own fonts don’t have.
In PDF-SDK
On iOS and Android the system’s fonts fill in letters a document’s font lacks; on the web you supply fonts for those scripts.
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