I have $15 to purchase 3PP PDF product, what should I buy and WHY?

I have the advantage to see a lot of 3PP products but not always able to purchase. But now I am interested in purchasing some 3PP that I normally might not pick up. I want to hear from the people that purchase 3PP products on what 3PP product I should pick up with my $15. I am open to anything from 3PP but make sure it is only PDF products only please.