``PFB'' stands for Printer Font Binary, and is a STORAGE FORMAT in which Adobe Type 1 font programs are usually distributed for IBM PC and compatibles. Many application programs support fonts in PFB format, and refer to them as ``downloadable''.
Macintosh uses a radically different binary storage format than PC. See below for some details.
PFB files usually appear in several sections, each section preceded by a binary header containing the type of the section (ASCII, binary, or end of file) and the length of the section. Because of the presence of the binary section headers, and the possible presence of binary data sections, PFB files cannot in general be sent directly to a PostScript printer. Application programs like dvips which use fonts in PFB format unpack the font into ASCII format before sending it to the printer. If you would like to use a font which is in PFB format, you must unpack the font to make a PFA (Printer Font ASCII) file. Adobe Systems supply a font downloader for PC's which turns the PFB format into PFA format on the fly as it's being downloaded.
``PFA'' stands for Printer Font ASCII, which is the unpacked version of a PFB file. In PFB, the data is stored as-is. In a PFA, any binary data present in a former PFB file is stored as ``ascii hex''-meaning each byte of binary data is turned into two ASCII characters representing the hexdecimal value.
Once you have the PFA file, just send it to the printer ahead of your file, and use the font like any other. There are several programs which can do the conversion from PFB to PFA for you. Try t1utils.
Details of the PFB format can be found in Adobe Technical Note #5040, ``Supporting Downloadable PostScript Fonts''.