Simple question...

When reading ebooks, do you prefer the file type to be .doc or .pdf?

I ask because I have a lot of ebooks I want to upload, but they're all .txt files. I figured this was the best place to ask...

I enjoyed Tad William's Memory, Sorrow and Thorn series. First two books are around 500-600 pages but the third one is a massive 1500 pages (conveniently split in two parts).