hi its easy to put pdfs on, but you can only put kindle books on a kindle ( or a device with a kindle app). For most ereaders the format is epub . you can get those from Kobo shop, Waterstones, whs. you can also borrow epub format from public library if your library has a subscription. I love my Kobo!
My dd has a kobo, there's a kobo ebook store, you can also download free epub books for it - the older classics and borrow e library books. I use calibre for converting from PDF to epub. I like my dds kobo - I have a kindle and an unable to read e library books.