Piotr offound the video below. You're looking at what was the cutting edge reading technology in 2007.
At some trade show that year Diginfo shot a video of a brand new reading app. It was designed to run on phones and it supported converting PDFs to DJVU format. The app used that obscure format because it was a compressed format that could reduce a PDF to under 20% of its original size. And yes, that thing above was a phone.
Think about how things have changed. These days pretty much every app supports PDF itself, and there's no need to compress the file. We have GBs of storage to spare, now.