On the first page of the PDF spec, the font dictionary specifies a base font of ""DDPEFM+Helvetica". I was expecting to see just "Helvetica". I had a look in the PostScript spec and I didn't find any mention of the "DDPEFM" prefix. What does it mean?
"For a font subset, the PostScript name of the font ... shall begin with a tag followed by a plus sign (+). The tag shall consist of exactly six uppercase letters; the choice of letters is arbitrary" i.e. the letters convey no useful information except to uniquely identify that subset in this particular PDF (since there may be several different subsets of the same font).
Subset fonts are made up specially for each PDF, by the software which creates the PDF. They usually contain exactly and only the glyphs needed for that PDF. By leaving out all the unused parts of fonts, PDF files are smaller. But the characters left are just a part of the regular Helvetica.
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