OK, right now browsing files happens in a narrow column on the left. Even if no file is selected the column stays narrow, and truncates file names that are too long for it. This is the first thing that should be fixed: use the whole screen for file browsing. Truncating file names is bad, so please don't do it.
When I select a PDF for preview, there is no possibility to change the zoom level. This is the second thing to be fixed: start with a view that shows the whole page, let us zoom in and out by double-tapping; and then let us close the file currently being previewed, exposing again the full screen file list.
The third shortcoming is the measly support for drag-n-drop.
Someone from support suggested I shared those "features" with the community. I really think these are very basic UI shortcomings that should be addressed by a core team dedicated to the best UX possible without us asking. But never mind, here we are, can You please fix the app?