Plenty of free e-books in Mobipocket format show up fine on Kindle.
DRM is not to prevent other files from being readable on the Kindle.
Use Mobipocket software to convert PDFs to that format so you can read them on Kindle.
The Kindle only allows the reading of Amazon DRM-protected content.
A revolutionary portable reader that wirelessly downloads books, newspapers, magazines and blogs to a crisp, high-resolution electronic paper display that looks and reads like real paper, even in bright sunlight.