Perhaps something like this? (untested):
selected_files = Dir.glob("*.pdf").select do |file|
mtime = File.mtime(file)
# if in a rails environment:
# (1.day.ago .. Time.now).cover?(mtime)
# if not in rails environment but want to use that code do this before that line:
# require 'active_support/all'
# else do the math:
# mtime > (Time.now - 86400) and mtime < Time.now
end