prawn：Prawn::Document::Internals#page_fonts

everyday Ruby 
#!/usr/bin/env ruby

require "prawn"

pdf = Prawn::Document.new

p pdf.page_fonts
p pdf.font

pdf.font "Times-Roman"
p pdf.page_fonts
p pdf.font

pdf.font_families.update(
   "MyTrueTypeFamily" => { :bold        => "foo-bold.ttf",
                           :italic      => "foo-italic.ttf",
                           :bold_italic => "foo-bold-italic.ttf",
                           :normal      => "foo.ttf" })
p pdf.page_fonts
p pdf.font

で、

{}
Prawn::Font::AFM< Helvetica: 12 >
{}
Prawn::Font::AFM< Times-Roman: 12 >
{}
Prawn::Font::AFM< Times-Roman: 12 >

？