In reply to BlownAway:
I'm still totally baffled by the need for this now. Surely one of the simplest modern solutions is to buy your own webspace, not very expensive, and then upload your pdfs into that? You could then upload them into a file deeply embedded into that webspace, e.g. www.yourwebspaceurl/misc/latest recipes/. That folder would then be password-protected (a password you could update as frequently as you like). They would already know the password or, if you're really paranoid, you could make a unique password for any latest file, and put a password hint at the top of that partic webpage, e.g, if you're communicating with an experienced climber, it might say something like 'surname of climber who did famous "suicidal" first ascent in the Ogwen valley in late 1940s, plus year of the ascent'. As soon as he/she's downloaded it, you can then delete it from the webspace, but you could already have it saved in some encrypted way, for ever?, eg. on Dropbox.
I'm just trying to give you the general drift of the modus operandi I'm working towards now. The thing is that what I've suggested requires no new technical competence on the part of the receiver.
PS. for extreme security you could convert you pdfs to jpgs before you upload them.
Post edited at 18:02