This is the case in which the creator of the PDF file didn't embed the font and Acrobat has to try to find the font installed on the user's system or use a substitution font. It you open a PDF file and look at the Fonts panel of Document Properties (Ctrl-D), for a particular font entry, the only place you would see Adobe Sans or Adobe Serif would be as the font listed as Actual Font. As Bo indicates, they are special substitution fonts used by Acrobat and some other Adobe applications.
They are not installed or otherwise available for normal application use. (1) A document could absolutely not have originally been created using Adobe Sans or Adobe Serif.
