`pdf_font`

Font embedding for PdfCanvas: the glyphs a document used, the font program carrying them, and the tables a PDF viewer needs to place and extract them.

A font goes into the file as a composite (Type0) font with the Identity-H encoding, so a glyph index is its own two-byte code and no cmap has to be re-encoded: the text operators carry the glyph indices the layout already chose (canvas.text.render’s shaping, with its ligatures and kerning), and the viewer draws exactly those. A ToUnicode CMap maps each used glyph back to the character it stood for, which is what makes the text selectable, searchable and copyable.

A TrueType (glyf) font is subset: the file keeps the tables a renderer reads outlines from – head, hhea, maxp, hmtx, loca, glyf, and the hinting tables cvt , fpgm, prep when present – and every unused glyph becomes an empty entry, with the components of a used composite glyph marked used with it. Glyph indices are unchanged, so CIDToGIDMap stays /Identity. A CFF-flavored OpenType font is embedded whole (FontFile3, subtype OpenType); subsetting a CFF program is a separate job.

The document carries the machine’s font, not a copy of the family

What is embedded is the bytes of the file font_discovery resolved on the machine that rendered, subset and reserialized. The tables listed above are copied verbatim out of it, glyf included, and the hinting programs cvt , fpgm and prep are copied whole. So the document is a function of which build of a family the machine had, not of the family name the caller asked for. Two machines with “the same font” from different packagings produce documents that differ in length: the glyph outlines agree, which is why raster output is identical, and the file bytes around them need not.

Nothing else in the document varies. There is no creation date and no /ID, and the /Producer is a fixed string, so the same font file on the same machine renders byte-identical documents run to run. A consumer gating on PDF bytes across machines is gating on the font installation; one gating on a single machine is gating on this package. The font program is most of a small document – a two-word label embeds about 53 KB of subset font, compressed to about 12 KB of a 14 KB file.