There are 3 basic ways to get things onto a Kindle:
- USB - once it's plugged into a PC, you can drag and drop stuff, use Calibre, etc...
- Email - each Kindle has a unique email address, and you can email it attachments, which it will add to it's library
- Buy from Amazon, send to Kindle.
As far as I know, you can't copy things over directly via WiFi, except by using email or the Amazon store.
Kindles do support PDF (but the reading experience is terrible) but they don't support Word Documents, etc... These are your native format choices:
- .mobi - Mobipocket eBook format (.amz is a derivative of this)
You can also use other formats by converting them to one of the supported formats. Calibre will automatically convert any files that it understands to the default Kindle format when you copy things across - and Amazons email service does some conversion too, at a cost:
Amazon offers an email-based service that will convert GIF, PNG and BMP graphics to AZW. Amazon will also convert HTML pages and Microsoft Word (DOC) or (DOCX) documents through the same email-based mechanism, which will send a Kindle-formatted file to the device via 3G for $0.15 per MB or via WiFi for free: from here
There are lots of other e-Ink devices available, apart from Kindle, which may well be much more flexible: https://en.wikipedia.org/wiki/List_of_e-book_readers
As for larger screens, e-Ink make a 9.7" screen, which is what the old Kindle DX used and is still available from some manufacturers.