There's absolutely no reason to purchase Amazon products with a Kindle if you don't want to. There are plenty of ebooks in various formats out there, and various simple bits of software which can convert non-Kindle formats to the Kindle format. Virtually all of the ebooks I've downloaded from Amazon - mainly classics - have been free.

As far as PDFs are concerned, I've experimented with loading chord sheets with some success. My particular Kindle is slightly too small to read some sheet music from easily, but a friend of mine uses the larger version as a sheet music reader for his concertina parts in sessions. It seems to work fine for him.

The Kindle is a useful adjunct in a house which is crammed with books from top to bottom in nearly every room (like mine), and I still buy regularly from AbeBooks and other online and high street booksellers. To be totally pro- or anti- a modern tool seems pointless to me. Use these devices with a bit of common sense, and take advantage of what they can do.