Often the fonts used are completely embedded in a PDF - i.e. the font embedded in the PDF is included with all available characters, although only a selection may be used.
The Fixup "Subset fonts" reduces the font in the PDF to the characters actually used. This procedure can often also reduce the file size by a considerable amount. In the attached, very simple example, for example, from 70 kB to only 19 kB.
The fonts are completely restructured, which often also corrects some font problems.