Deleting a document from PDF Viewer can be achieved in a couple of ways.
If you want to delete multiple documents at once:
Tap "Select" on the top right corner of the app.
Select the documents and folders you want to delete from PDF Viewer by tapping on them, marking them with a blue checkmark.
Tap on the bottom right corner button (Trash Can icon on iPhone, "Delete" button on iPad) to delete them.
Note that the "Select" button is only available in the Browse tab.
If you want to delete a single document:
Tap and hold the document you want to delete.
Select "Delete" from the contextual menu that's displayed after a few seconds (you may need to navigate the menu by tapping the right arrow on the menu).
You can delete items individually from both the Recents and Browse tabs.