How to deliver digital products to iPad and iPhone clients

What I would do:

Create an application like the Audible/iTunes app. User can shop for new stuff, and both download and view previous purchases. The app would need to access your client database and purchase history. Depending on your budget, this may not be a realistic option. But long term, that is what I would aim for.

Most oDesk developers are happy to do a quote over Skype.

Otherwise, you can start an iBooks account and sell .pdfs that way. Apple will take a cut. You can do a similar thing with the .mp3's and Podcasts.

Or if you'd really like to keep a trim budget, you can direct the user to add stuff manually, provided they have access to iTunes and a computer.

If the purchaser does have access to a computer with iTunes, they can add the .mp3 and sync across. i.e. How to Add Downloaded MP3s to iTunes

.Pdf - If they have the Pages iOS application they can view these in pages saved via iCloud. Other apps would probably do this, Pages is just the one that I know definitely does.

If they don't have access to a computer with iTunes, a bit primitive - email the contents through and they can view .pdfs that way. Might also be able to play .mp3's, except if the .mp3 is > 40mb and they're not connected to Wifi (I think that's about right). Not really a professional look this way though, but a workaround nonetheless.

Hope that helps!