I cannot see many paying for a pdf document on a monthly basis unless its really of a very good quality but even then the numbers will be very low.

I also think you will have a better chance at success if you convert your idea into a website and then to get some advertisers to advertise on the site. You can maybe even include a faucet on the website in order to attract some traffic.

There is just so much more you can do with a website and it will be easier to create an income from that than it would be to get it from a monthly pdf document/magazine.