RE #1: A PDF file is just like any other file, so it can be copied in its entirety. So #1 is out.
RE #2: Counterproductive. Mags prosper as verifiable eyeballs increase due to advertising. If people won't subscribe to the print version, they're not going to subscribe to a more expensive electronic version.
RE #3: Promotional, with the objective of increasing print circulation, therefore it's irrelevant.
RE #4: Counterproductive, unless back issues are only offered to print subscribers.
That brings me to #5, which I'm originating:
5) Destroy the value of PDF files by making the opportunity cost of copying or distributing them prohibitive. This is one way it could work:
The first thing that happens when a download commences is that a cookie is written to the subscriber's computer by an ASP page that first verifies that the downloader is authorized through logins, passwords, etc.
Each issue is broken up into about 10+ discrete PDF files. Each file would have an internal link to the succeeding file.
For example, say the first file ends with a link to the 2nd file, which begins with the next page. If the 2nd file is already in the cache, it will open quickly and seamlessly. If not, the download URL will be navigated to, the cookie read, the download commenced automatically, and the 2nd file displayed slower but still seamlessly. If there's no cookie, the login page will be displayed, which will thwart the pirate unless the subscriber has provided the login information to the pirate.
Here's where the magic comes in. Each subscriber has an IP address tied to a specific ISP in a geographic area. The ASP page records the IP address in a SQL database correlated to the subscriber. If all 4 elements of the 4 part IP address match, that's your subscriber, so commence the download. If the first 2-3 elements don't match, odds are somebody else is trying to download. The ASP page refuses to initiate downloading and redirects the visitor to a "my account" page, which will transfer the subscription to the new user after the "different" subscription logon information is provided. The original "subscriber" will then be cut off. There won't be very many subscribers "sharing" their logon information....
If you can't lick 'em, trick 'em.