I have Pocketbook Inkpad 2 which has a 8" screen, but a bit older software. I also have Touch HD, which has a 6" screen, but its software is as new as in Inkpad 3, the newest Pocketbook.
I have never had a Kobo thus far, but everything I have heard about it tells me that the pdf reading experience is very bad on Kobo. If you read pdf files, Pocketbook is a better pick in terms of software (particularly the inbuilt reader app).
In terms of hardware, there should be no notable difference between Pocketbook Inkpad 3 and Kobo Aura One, except that Pocketbook comes with buttons, page turn keys. I like page turn keys very much. On Kobo you have to rely entirely on the touchscreen.
Another notable difference between the two is that Pocketbook's cover is about three times cheaper. I consider the cover an absolute necessity, particularly on bigger ereaders. (The good thing with Onyxes is that covers are included in the price, a minor point but worth considering).
If you still buy a Kobo Aura One (because why not, it's water resistant after all, and that might come in handy), then here's how to make it usable with pdf
https://www.youtube.com/watch?v=i2oSOeAnD10