If you are running a version of windows you will have a free email client as part of that. In XP its Outlook Express, in vista, windows 7 it's called windows mail. That will quite happily send 300 emails, just make sure the file you are attaching isn't silly big, Typically you wouldn't want to be sending a pdf much bigger than 1-2mb. You can of course send bigger its just not necessarily a good idea, especially to 300 recipients.
E-I