Documents is free though. I didn't get PDF Expert because I didn't need it, so I was not aware it shared so much functionality with PDF Expert. Does it also allow for browser downloads?
Regardless, if PDF Expert is a superset of Documents functionality than it is the same point, just a matter of whether the user wants to pay for it.
Yes, PDF Expert has browser downloads.
And of course, if one doesn't need PDF annotating, Documents is the obvious choice. Especially because Documents is free.