Niche Digital publishing

I have been asked to look into a platform or system to digitally deliver Magazines via secured pdf or some online app system. From previous forays, these tend to be enterprise systems and far too costly for my needs.

Can anyone suggest a system that might suite Niche Digital publishing?

I help publish a niche quarterly magazine which is sold on a subscription only basis via a web site the magazine is print and distributed around the world. We also sell pdfs which can only be purchased in sets of 10 or as a whole catalogue of back issues and only on CD or DVD.

Basically, the magazine relies totally on its subscription basis, there is limited advertising which generates negligible revenue. The mag sells for £2.50 plus postage so not a big cash generator it’s more a labour of love to the editor but in its area is highly respected.

Production of the magazine in what ever format will not be an issue, it’s more the secure delivery of the magazine in a manner that does not under mind the subscription model of the magazine. Any thoughts or recommendations appreciated.