Now that font embedding has been added with LibO 18.104.22.168, this feature should be added to Lib0 Help as, for example:
LibreOffice Help -> Index tab -> fonts -> embedding ->
or, in other words:
Open File menu -> Properties -> Font tab -> Check "Embed fonts in the document"
Also, it might be a good idea to distinguish embedding fonts in PDF documents from embedding fonts in Writer, Calc and Impress documents
Confimed in LO 22.214.171.124
It seems that it is missing in the online as well as in the offline help.
Note that the path might change. The Font tab really should not be present (see bug 64357).
Olivier Hallot committed a patch related to this issue.
It has been pushed to "master":
tdf#70141 Add help page for font embedding