PDF Viewer for iOS allows documents to be shared with other people and apps via the standard share sheet.
To do so, open PDF Viewer and make sure you're in the Browse tab. Then:
Tap "Select" on the top-right corner.
Select the documents and folders you want to share by tapping on them, marking them with a blue checkmark.
Tap on the bottom left corner button (Share icon on iPhone, "Share" button on iPad) to share them.