I don't think the kindle does PDF's... it's really geared towards downloading from the amazon store, and so you should probably avoid that one.
none of them do PDF's very well because PDF's are formatted to 8.5/11 typically... but if she can resize them for a smaller device than it'll be fine on something like the cybook or hanlin or iliad or...sony? I'm not sure about the sony and PDF either.
according to the comparison matrix here:
the hanlin v3 is the only one that will display her .doc files. It looks like they all do PDF except the sony and the stareread.
The iliad has the biggest screen so is best for full sized PDF.
the iliad has the least battery life (touch screen and wifi take up battery).. but even that will give her 12 hours I think.
The others all like to say something like "about a month reading 300 pages per day" .... it's something like 7,000-10,000 page refreshes. They all use the same screens and similar processors so it's mainly only software differences and what particular li-ion battery is used.
It's too bad iliad is still the only large screen one. I don't really like the idea of all the extra features in the iliad.