Well, at least it isn't nose-picking time!
OK, as a clarification, PDF::Builder includes metrics for a number of fonts, but not the font files themselves. That depends on the OS and anything the PC owner has installed.
Regarding substitutions, if, for instance, Adobe Acrobat Reader (is that the same as Adobe Reader? according to Adobe, yes) decides to substitute Arial for Helvetica, are the metrics exactly the same, or just "close"? If PDF::Reader (or text processing based on it) has carefully justified a line of type in Helvetica, will the Arial text fit exactly the same (or at least, no noticeable difference)?
And what happens when I request, say, Bank Gothic and the font itself isn't available on the reader (but the metrics are, so the PDF file generation is successful)? Will the reader try to substitute something else, or just give a lot of errors?