You didn't state whether or not this is the pc that you use to sync your iphone on. I'm going to assume that it is, correct me if I'm wrong. I just add pdf to itunes in the book section then sync it over to ibooks using itunes. Easy Peezey. I'm pretty sure as well if you have adobe reader installed on your iphone you can use the apps section, scroll to the bottom, then use that section of itunes to put any pdf that you wish to read directly onto your device into the adobe reader app.
If this isn't a computer that has itunes installed, or that you dont' sync with then I'm sure the other suggestions work fine. If it is your personal computer and the one you sync with this is the absolute easiest, cheapest way that I have found to do what your asking.