Fonts are the thing PDF gets praised for and complained about in the same breath. When they work, your document looks identical everywhere. When they don't, you get Times New Roman on a CV you spent hours typesetting. Here's the mechanism behind both outcomes.
Embedding: fonts as luggage
When you export to PDF with embed fonts ticked, the file carries copies of the fonts it uses. Anyone opening it sees the original typeface even if they don't have it installed. This is why a fully-embedded PDF looks the same on every device. It also adds size — usually 100–500 KB per font. Most modern export tools embed by default; older or budget tools sometimes don't.
Subsetting: only what's needed
Embedding the entire Helvetica family for a document that only uses 80 characters would be wasteful. Subsetting strips embedded fonts down to just the glyphs the document actually uses. The downside: if you later edit the PDF and add a character that wasn't originally in the file (an é, an em-dash), the font might not have it and your editor substitutes. Sometimes you can spot a subset because the font name starts with random letters.
Substitution: when the font isn't there
If a font isn't embedded and isn't installed on your computer, the viewer picks the closest match. Helvetica becomes Arial, Garamond becomes Times, fancy display fonts become whatever's handy. The document still works; it just looks different. For documents that matter, always embed at export. For documents you'll keep editing, stick to fonts your editor knows it has.
FAQ
Why does my PDF say 'font not embedded'?
The export skipped embedding for that font — usually because it's a system font the tool assumed everyone has, or because of licensing restrictions. Re-export from the source with 'embed all fonts' selected.
Can a PDF use a font I don't own?
If the font is embedded, yes — you see it without owning it. But you can't extract it for use elsewhere without a licence. Subsetting is partly a copyright workaround.
Will embedded fonts make the file slower?
Negligibly. PDF readers load fonts on demand. A few hundred KB doesn't slow modern devices. The size matters more for email attachments than for viewing.
Embedded fonts are the difference between 'looks great' and 'looks fine'. Set 'embed fonts' on at export — and Flint preserves them on save.