Hi welcome to mobileread
Gosh lots of questions - I hope they all get answered for you!
If you specifically want to read .pdf files, all the ebook reading devices I'm aware of have some issues with them. Having viewed pdfs myself on a whole variety of screen sizes, I say the bigger the screen size the better generally.
But perhaps you can find the books you want in .epub format which is the ebook standard format and this will open up many more possibilities to you in terms of which device to get.
I have a netbook which I carry around the house on a tray. It is great for Internet use and email but I can't comfortably read on it for long before the screen starts making my eyes ache. For long periods of reading, an e-ink ereader screen is very comfortable.
Anyway hope you get more advice soon.