New eBook Reader from Adobe
I highly recommend you download Adobe's relatively new ebook reader: Adobe - Digital Editions. It makes reading pdf ebooks very easy on the eyes. I used to hate reading ebooks in pdf format because it was somewhat cumbersome. Now that I've been using Adobe Digital Editions, I find myself preferring to read pdf copies to physical copies.
DesiringGod has released some of Pipers books and other resources for free in pdf format, Sovereign Grace has many pdf resources, and Crossway often will send you a pdf copy of a book if you purchase it directly from them (usually a little more expensive though).