This is the Adobe Acrobat user Community, so all I can tell you about is Acrobat - Adobe has a lot of different software products.
The way this usually works is not that you decide to become an expert in any technology, it's the other way around: You pick a technology and work with it for long enough to be considered an expert. In my case that's almost 20 years that I've worked with and around the PDF format. Yes, there are certifications that you can add to your resume, but at the end what counts is your experience. And that has to encompass more than just experience with the application, this needs to be experience in the "real world" using the application, understanding how it interacts with other parts of a workflow. It also requires you to know about the industry or industries where this application is used.
So, don't set out to become an expert, do the work and you will become an expert automatically after a few years.
And once you do have that experience and some name recognition, you will be able to ear a living.
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com