Why don't resume improvement articles talk about links on PDF resumes?

Based on my experience, links are relevant to the industry you're in and/or the job you are pursuing. Immediately what comes to mind are IT professionals. They tend to include multiple links on their résumé to different projects they've created or have contributed to in the IT community.

I always assumed that any organization to which I applied for a job would try to develop some intel about me as a candidate before an interview. I’m sure there are a few other Tony Lambs on the Internet, so to keep them focused on the right one, I would include links to my email, blogs, or Twitter account, as well as hyperlinks to my college and previous work experience, with the expectation that my resume would be viewed electronically. It creates a system of information for potential employers to reference.

It's not a bad idea to have on there. I think it depends on the field you are in and what your goal is in having the links. For example, a photographer having a link to his/her photos makes sense.