Depending on what you like, you can find a large selection of free books. I don't buy very many e-books, most of my reads are freebies or library loans. Does your library support lending of e-books? Do you like many of the public domain reads or self-published books? There are lots of freebies out there, and you don't have to limit yourself. It'd be a shame to buy an e-reader and then just use pdfs, ick.
PDFs I find are not good for text-based literature. They're fine for picture heavy documents, but none of the e-readers I've used so far are a preference for pdfs.