What does Embedded fonts mean for your PDF?
A PDF that embeds its fonts renders identically everywhere. One that only names them relies on the reader having something similar, which is how a careful layout turns into Courier on someone else’s laptop.
Most files embed a subset: only the characters actually used, which keeps the file small. It is also why you cannot install a font extracted from a PDF — the glyphs for letters nobody typed are not in there.
Subsetting is why adding text to an existing PDF sometimes needs a new font embedded: the letters you type may not exist in the subset already present.
How do you check which fonts a PDF carries?
Open the document properties and look at the Fonts tab. Each entry is listed with its type and, crucially, whether it says (Embedded) or (Embedded Subset). A font named with neither is not in the file, and the reader is substituting something on the fly.
Substitution is why a layout that looked right on your machine arrives at a printer or a client as Courier or Times, with line breaks in new places and tables that no longer line up. Nothing was corrupted — the file simply asked for a typeface that was not there.
If a document is going anywhere important, check this before sending rather than after. Re-exporting with fonts embedded takes a minute; a reprint does not.