You can also use Win Explorer to drag/drop to the folder mentioned above.
And just for more info: You canNOT load books for the kobo app - the books must be purchased from kobo (on pb or other method) and then downloaded directly on the pb using the kobo app. Someone mentioned that if the books are drm-free, even if purchased from kobo, the pb canNOT read them - I don't know if this is correct or not.
Kobo app only reads Kobo library books, it won't sideload like the Kobo Vox native app which will read epubs. No Kobo app reads mobi but Calibre converts mobi to epub.
Playepub app works well for side loaded epubs. Overdrive works for drm protected adobe epubs.