Hi, great question. Lots of things have already been written on this subject and having the right portfolio medium when applying for jobs is sometimes tricky to get right. In an ideal situation you should have both.
An online personal website or portfolio showcase like Adobe's Myportfolio to get potentional employers to look at what you're capable of. The attention span of HR managers is short and you have to aim to convince them of your skills in a short amount of time. PDF's often get bulky quick and result in slow load times, portfolio sites (generally) load quick, create great oversight, and aren't all that dificult to set up. Employers also aren't that keen to download files from strangers.
However PDF's have the advantage that you can tailor your portfolio to the specific needs of potentional employers, but personally I keep this for an in person interview.
For in person interviews I always bring a print version and a digital version (of the same document). Here you can ellaborate more on process and design decisions and prove why you are the right fit for the team. I would stress to always bring both of these, even if they say in advance that a digital version will sufice. Print versions are easier to discuss one on one and they are especially usefull when displays crash or scale incorrectly. It's also always usefull to have spares or a backup plan.
In short, the first step is to get your foot in the door and convince HR, second step is to convince your interviewer. Both steps have a different approach and your portfolio should be tailored to this.
This is my 2c as a recent grad in the EU.