Hey Gail,
The first thought is Dropbox and having a link to the folder or document. But really you can substitute the name Dropbox for any other place where you can put your file and only those who you want to have access to it, can have access to it.
Brainstorming a bit:
Dropbox, Box, Google Drive (which it could even be a restricted Google site you create just for your files...the down side may be branding consistency), Does Flickr allow for restricted access to folders, Depending on what you are using for a website you could create a restricted page to house the lead magnet, also saw something called beacon.by37 for lead magnets.
Just a few thoughts but I'm sure others can give you even more.