Hi,

You could create a page for downloads with view permissions set to "Authenticated Users" role. Optionally you could make the page not included in the menu or site map, it would still be accessible by url if the user knew the url.

You could put a shared files feature on this page and load your pdf files into. Then you could put a link to the download url for the file in your blog post.

There is no way to hide the link in your post but if an unauthenticated user clicks it they will be prompted to login before they can download.

The shared files will also keep track of download counts.

Hope it helps,

Joe