Sony PRS-T1 ? it maybe too small for PDF but I think it fits your requirement and also it is very cheap, a refurbished Kindle Fire is also an interesting idea if you are living in the US especially for
the refurbished one. Since it is cheap and it comes with a one year warranty. Or if you don't mind about dated technology, Kindle DX Graphite is also a good choice, it maybe expensive, but it can play MP3 as well as PDF reader with a big screen. The only problem about DXG is I don't quite sure if you can use DXG with Linux or not.