title_body: Kobo Aura One or the new 7 inch Kindle Oasis for PDF files?
upvoted_answer: Ever since the time I originally asked this question, I have bought both a Sony DPT-RP1 and an old Kindle DX.
  
  The DPT-RP1 is really excellent. Thanks to its screen size, almost any conventional PDF book or article are perfectly readable.  But this has little to do with the answer. In fact I more recently bought an old Kindle DX because it was cheap and I wanted to try it. I am still amazed by how capable this old thing is. Its display quality is surprisingly good considering its age, and having a lower resolution than the DPT-RP1 has rarely been an issue.
  
  The Kindle DX has a 9.7 inch-screen so it is much smaller than that of the DPT-RP1. However, aside from some rare exceptions, almost all of my PDF books are perfectly readable on it. Issues arise when reading two-column formatted scientific papers where the font size is small and the original paper size is A4. These are the bane of the DX because you have to either read in landscape mode and constantly scroll using physical buttons; or zoom in and again, scroll up-down and right-left. So when most PDF files are formatted this way, indeed, as stated in a comment, the DX is "barely tolerable".
  
  I do not own a Kobo Aura One nor a Kindle Oasis, however, my experience with the DPT-RP1 and DX drive me to the conclusion that neither of those devices are good enough to deal with common PDF files because of their screen size.