You mean Documents by Readdle? There are so many PDF apps for iOS it's hard to keep track of them all.
If you do mean Documents by Readdle, it's something like a "poor cousin" (or you might say a free preview, with limited functionality) of Readdle's flagship app,
PDF Expert. I can't recommend PDF Expert highly enough. GoodReader is excellent as well, but PDF Expert both looks better and seems, to me, a bit smarter in the reading options it offers.
(For example, compare the two "Night Mode" options in GoodReader and PDF Expert -- both are nice, but only PDF Expert's "Night Mode" is a genuine "Night Mode".)