Welcome to mobileread, MO74.
You might be better off posting your question in the "Which one should I buy" section.
If you are interested in public domain books, you can find a lot of them right here in mobileread, lovingly formatted by our members, in various popular formats.
The sony handles pdf files quite well. If the paper size of the pdf is large you will have to reflow, maybe losing some formatting along the way, but nothing serious if there are not a lot of graphics and tables. Most public domain books can be found in many other formats though, like epub, which is much better supported.
You can read any non-drm mobi book on the kindle (maybe pdf too, I'm not sure, check the reader matrix). The restrictions only apply for when you buy drm ebooks (like commercial new releases). For those you can only use the amazon store.
I don't find a computer screen comfortable for reading, and I know that at least for me, buying an ebook reader (or three) was one of my best buying decisions ever. I do 90% of my reading on my prs-600 now.
Good luck with your choice!